diff --git a/modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache b/modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache
index f2cfe345f1f6..7f1030626892 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/apis.mustache
@@ -107,7 +107,7 @@ export class {{classname}} extends runtime.BaseAPI {
     async {{nickname}}Raw({{#allParams.0}}requestParameters: {{#prefixParameterInterfaces}}{{classname}}{{/prefixParameterInterfaces}}{{operationIdCamelCase}}Request, {{/allParams.0}}initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<{{{returnType}}}{{^returnType}}void{{/returnType}}>> {
         {{#allParams}}
         {{#required}}
-        if (!runtime.exists(requestParameters, '{{paramName}}')) {
+        if (requestParameters['{{paramName}}'] == null) {
             throw new runtime.RequiredError(
                 '{{paramName}}',
                 'Required parameter "{{paramName}}" was null or undefined when calling {{nickname}}().'
@@ -120,7 +120,7 @@ export class {{classname}} extends runtime.BaseAPI {
 
         {{#queryParams}}
         {{#isArray}}
-        if (runtime.exists(requestParameters, '{{paramName}}')) {
+        if (requestParameters['{{paramName}}'] != null) {
             {{#isCollectionFormatMulti}}
             queryParameters['{{baseName}}'] = requestParameters['{{paramName}}'];
             {{/isCollectionFormatMulti}}
@@ -131,7 +131,7 @@ export class {{classname}} extends runtime.BaseAPI {
 
         {{/isArray}}
         {{^isArray}}
-        if (runtime.exists(requestParameters, '{{paramName}}')) {
+        if (requestParameters['{{paramName}}'] != null) {
             {{#isDateTimeType}}
             queryParameters['{{baseName}}'] = (requestParameters['{{paramName}}'] as any).toISOString();
             {{/isDateTimeType}}
@@ -161,13 +161,13 @@ export class {{classname}} extends runtime.BaseAPI {
         {{/bodyParam}}
         {{#headerParams}}
         {{#isArray}}
-        if (runtime.exists(requestParameters, '{{paramName}}')) {
+        if (requestParameters['{{paramName}}'] != null) {
             headerParameters['{{baseName}}'] = {{#uniqueItems}}Array.from({{/uniqueItems}}requestParameters['{{paramName}}']{{#uniqueItems}}){{/uniqueItems}}!.join(runtime.COLLECTION_FORMATS["{{collectionFormat}}"]);
         }
 
         {{/isArray}}
         {{^isArray}}
-        if (runtime.exists(requestParameters, '{{paramName}}')) {
+        if (requestParameters['{{paramName}}'] != null) {
             headerParameters['{{baseName}}'] = String(requestParameters['{{paramName}}']);
         }
 
@@ -238,7 +238,7 @@ export class {{classname}} extends runtime.BaseAPI {
 
         {{#formParams}}
         {{#isArray}}
-        if (runtime.exists(requestParameters, '{{paramName}}')) {
+        if (requestParameters['{{paramName}}'] != null) {
             {{#isCollectionFormatMulti}}
             requestParameters['{{paramName}}'].forEach((element) => {
                 formParams.append('{{baseName}}', element as any);
@@ -251,7 +251,7 @@ export class {{classname}} extends runtime.BaseAPI {
 
         {{/isArray}}
         {{^isArray}}
-        if (runtime.exists(requestParameters, '{{paramName}}')) {
+        if (requestParameters['{{paramName}}'] != null) {
             {{#isPrimitiveType}}
             formParams.append('{{baseName}}', requestParameters['{{paramName}}'] as any);
             {{/isPrimitiveType}}
diff --git a/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache b/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache
index df0df852beb9..7a5289c35144 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache
@@ -1,4 +1,4 @@
-import { exists, mapValues } from '../runtime{{importFileExtension}}';
+import { mapValues } from '../runtime{{importFileExtension}}';
 {{#hasImports}}
 {{#imports}}
 import type { {{{.}}} } from './{{.}}{{importFileExtension}}';
@@ -38,7 +38,7 @@ export function {{classname}}FromJSON(json: any): {{classname}} {
 
 export function {{classname}}FromJSONTyped(json: any, ignoreDiscriminator: boolean): {{classname}} {
     {{#hasVars}}
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
 {{#discriminator}}
@@ -58,36 +58,36 @@ export function {{classname}}FromJSONTyped(json: any, ignoreDiscriminator: boole
         {{#vars}}
         {{#isPrimitiveType}}
         {{#isDateType}}
-        '{{name}}': {{^required}}!exists(json, '{{baseName}}') ? undefined : {{/required}}({{#required}}{{#isNullable}}!exists(json, '{{baseName}}') ? null : {{/isNullable}}{{/required}}new Date(json['{{baseName}}'])),
+        '{{name}}': {{^required}}json['{{baseName}}'] == null ? undefined : {{/required}}({{#required}}{{#isNullable}}json['{{baseName}}'] == null ? null : {{/isNullable}}{{/required}}new Date(json['{{baseName}}'])),
         {{/isDateType}}
         {{#isDateTimeType}}
-        '{{name}}': {{^required}}!exists(json, '{{baseName}}') ? undefined : {{/required}}({{#required}}{{#isNullable}}!exists(json, '{{baseName}}') ? null : {{/isNullable}}{{/required}}new Date(json['{{baseName}}'])),
+        '{{name}}': {{^required}}json['{{baseName}}'] == null ? undefined : {{/required}}({{#required}}{{#isNullable}}json['{{baseName}}'] == null ? null : {{/isNullable}}{{/required}}new Date(json['{{baseName}}'])),
         {{/isDateTimeType}}
         {{^isDateType}}
         {{^isDateTimeType}}
-        '{{name}}': {{^required}}!exists(json, '{{baseName}}') ? undefined : {{/required}}json['{{baseName}}'],
+        '{{name}}': {{^required}}json['{{baseName}}'] == null ? undefined : {{/required}}json['{{baseName}}'],
         {{/isDateTimeType}}
         {{/isDateType}}
         {{/isPrimitiveType}}
         {{^isPrimitiveType}}
         {{#isArray}}
         {{#uniqueItems}}
-        '{{name}}': {{^required}}!exists(json, '{{baseName}}') ? undefined : {{/required}}{{#required}}({{#isNullable}}!exists(json, '{{baseName}}') ? null : {{/isNullable}}{{/required}}new Set((json['{{baseName}}'] as Array<any>).map({{#items}}{{datatype}}{{/items}}FromJSON))),
+        '{{name}}': {{^required}}json['{{baseName}}'] == null ? undefined : {{/required}}{{#required}}({{#isNullable}}json['{{baseName}}'] == null ? null : {{/isNullable}}{{/required}}new Set((json['{{baseName}}'] as Array<any>).map({{#items}}{{datatype}}{{/items}}FromJSON))),
         {{/uniqueItems}}
         {{^uniqueItems}}
-        '{{name}}': {{^required}}!exists(json, '{{baseName}}') ? undefined : {{/required}}({{#required}}{{#isNullable}}!exists(json, '{{baseName}}') ? null : {{/isNullable}}{{/required}}(json['{{baseName}}'] as Array<any>).map({{#items}}{{datatype}}{{/items}}FromJSON)),
+        '{{name}}': {{^required}}json['{{baseName}}'] == null ? undefined : {{/required}}({{#required}}{{#isNullable}}json['{{baseName}}'] == null ? null : {{/isNullable}}{{/required}}(json['{{baseName}}'] as Array<any>).map({{#items}}{{datatype}}{{/items}}FromJSON)),
         {{/uniqueItems}}
         {{/isArray}}
         {{#isMap}}
-        '{{name}}': {{^required}}!exists(json, '{{baseName}}') ? undefined : {{/required}}({{#required}}{{#isNullable}}!exists(json, '{{baseName}}') ? null : {{/isNullable}}{{/required}}mapValues(json['{{baseName}}'], {{#items}}{{datatype}}{{/items}}FromJSON)),
+        '{{name}}': {{^required}}json['{{baseName}}'] == null ? undefined : {{/required}}({{#required}}{{#isNullable}}json['{{baseName}}'] == null ? null : {{/isNullable}}{{/required}}mapValues(json['{{baseName}}'], {{#items}}{{datatype}}{{/items}}FromJSON)),
         {{/isMap}}
         {{^isArray}}
         {{^isMap}}
         {{^isFreeFormObject}}
-        '{{name}}': {{^required}}!exists(json, '{{baseName}}') ? undefined : {{/required}}{{datatype}}FromJSON(json['{{baseName}}']),
+        '{{name}}': {{^required}}json['{{baseName}}'] == null ? undefined : {{/required}}{{datatype}}FromJSON(json['{{baseName}}']),
         {{/isFreeFormObject}}
         {{#isFreeFormObject}}
-        '{{name}}': {{^required}}!exists(json, '{{baseName}}') ? undefined : {{/required}}json['{{baseName}}'],
+        '{{name}}': {{^required}}json['{{baseName}}'] == null ? undefined : {{/required}}json['{{baseName}}'],
         {{/isFreeFormObject}}
         {{/isMap}}
         {{/isArray}}
@@ -102,11 +102,8 @@ export function {{classname}}FromJSONTyped(json: any, ignoreDiscriminator: boole
 
 export function {{classname}}ToJSON(value?: {{classname}} | null): any {
     {{#hasVars}}
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         {{#parent}}...{{{.}}}ToJSON(value),{{/parent}}
@@ -117,13 +114,13 @@ export function {{classname}}ToJSON(value?: {{classname}} | null): any {
         {{^isReadOnly}}
         {{#isPrimitiveType}}
         {{#isDateType}}
-        '{{baseName}}': {{^required}}!exists(value, '{{name}}') ? undefined : {{/required}}({{#required}}{{#isNullable}}!exists(value, '{{name}}') ? null : {{/isNullable}}{{/required}}(value['{{name}}']{{#isNullable}} as any{{/isNullable}}).toISOString().substring(0,10)),
+        '{{baseName}}': {{^required}}value['{{name}}'] == null ? undefined : {{/required}}({{#required}}{{#isNullable}}value['{{name}}'] == null ? null : {{/isNullable}}{{/required}}(value['{{name}}']{{#isNullable}} as any{{/isNullable}}).toISOString().substring(0,10)),
         {{/isDateType}}
         {{#isDateTimeType}}
-        '{{baseName}}': {{^required}}!exists(value, '{{name}}') ? undefined : {{/required}}({{#required}}{{#isNullable}}!exists(value, '{{name}}') ? null : {{/isNullable}}{{/required}}(value['{{name}}']{{#isNullable}} as any{{/isNullable}}).toISOString()),
+        '{{baseName}}': {{^required}}value['{{name}}'] == null ? undefined : {{/required}}({{#required}}{{#isNullable}}value['{{name}}'] == null ? null : {{/isNullable}}{{/required}}(value['{{name}}']{{#isNullable}} as any{{/isNullable}}).toISOString()),
         {{/isDateTimeType}}
         {{#isArray}}
-        '{{baseName}}': {{#uniqueItems}}{{^required}}!exists(value, '{{name}}') ? undefined : {{/required}}{{#required}}{{#isNullable}}!exists(value, '{{name}}') ? null : {{/isNullable}}{{/required}}Array.from(value['{{name}}'] as Set<any>){{/uniqueItems}}{{^uniqueItems}}value['{{name}}']{{/uniqueItems}},
+        '{{baseName}}': {{#uniqueItems}}{{^required}}value['{{name}}'] == null ? undefined : {{/required}}{{#required}}{{#isNullable}}value['{{name}}'] == null ? null : {{/isNullable}}{{/required}}Array.from(value['{{name}}'] as Set<any>){{/uniqueItems}}{{^uniqueItems}}value['{{name}}']{{/uniqueItems}},
         {{/isArray}}
         {{^isDateType}}
         {{^isDateTimeType}}
@@ -136,14 +133,14 @@ export function {{classname}}ToJSON(value?: {{classname}} | null): any {
         {{^isPrimitiveType}}
         {{#isArray}}
         {{#uniqueItems}}
-        '{{baseName}}': {{^required}}!exists(value, '{{name}}') ? undefined : {{/required}}({{#required}}{{#isNullable}}!exists(value, '{{name}}') ? null : {{/isNullable}}{{/required}}Array.from(value['{{name}}'] as Set<any>).map({{#items}}{{datatype}}{{/items}}ToJSON)),
+        '{{baseName}}': {{^required}}value['{{name}}'] == null ? undefined : {{/required}}({{#required}}{{#isNullable}}value['{{name}}'] == null ? null : {{/isNullable}}{{/required}}Array.from(value['{{name}}'] as Set<any>).map({{#items}}{{datatype}}{{/items}}ToJSON)),
         {{/uniqueItems}}
         {{^uniqueItems}}
-        '{{baseName}}': {{^required}}!exists(value, '{{name}}') ? undefined : {{/required}}({{#required}}{{#isNullable}}!exists(value, '{{name}}') ? null : {{/isNullable}}{{/required}}(value['{{name}}'] as Array<any>).map({{#items}}{{datatype}}{{/items}}ToJSON)),
+        '{{baseName}}': {{^required}}value['{{name}}'] == null ? undefined : {{/required}}({{#required}}{{#isNullable}}value['{{name}}'] == null ? null : {{/isNullable}}{{/required}}(value['{{name}}'] as Array<any>).map({{#items}}{{datatype}}{{/items}}ToJSON)),
         {{/uniqueItems}}
         {{/isArray}}
         {{#isMap}}
-        '{{baseName}}': {{^required}}!exists(value, '{{name}}') ? undefined : {{/required}}({{#required}}{{#isNullable}}!exists(value, '{{name}}') ? null : {{/isNullable}}{{/required}}mapValues(value['{{name}}'], {{#items}}{{datatype}}{{/items}}ToJSON)),
+        '{{baseName}}': {{^required}}value['{{name}}'] == null ? undefined : {{/required}}({{#required}}{{#isNullable}}value['{{name}}'] == null ? null : {{/isNullable}}{{/required}}mapValues(value['{{name}}'], {{#items}}{{datatype}}{{/items}}ToJSON)),
         {{/isMap}}
         {{^isArray}}
         {{^isMap}}
diff --git a/modules/openapi-generator/src/main/resources/typescript-fetch/modelOneOf.mustache b/modules/openapi-generator/src/main/resources/typescript-fetch/modelOneOf.mustache
index 9e6b5d497956..96cac044f519 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/modelOneOf.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/modelOneOf.mustache
@@ -17,7 +17,7 @@ export function {{classname}}FromJSON(json: any): {{classname}} {
 }
 
 export function {{classname}}FromJSONTyped(json: any, ignoreDiscriminator: boolean): {{classname}} {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
 {{#discriminator}}
@@ -36,11 +36,8 @@ export function {{classname}}FromJSONTyped(json: any, ignoreDiscriminator: boole
 }
 
 export function {{classname}}ToJSON(value?: {{classname}} | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
 {{#discriminator}}
     switch (value['{{discriminator.propertyName}}']) {
diff --git a/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache b/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
index 0db3b0ac14eb..991ac373495b 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/runtime.mustache
@@ -299,13 +299,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-{{^withoutRuntimeChecks}}
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-{{/withoutRuntimeChecks}}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/apis/DefaultApi.ts b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/apis/DefaultApi.ts
index fd8a011fc7cd..38da4f052ba3 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/apis/DefaultApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/apis/DefaultApi.ts
@@ -34,7 +34,7 @@ export class DefaultApi extends runtime.BaseAPI {
     /**
      */
     async listRaw(requestParameters: ListRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Club>> {
-        if (!runtime.exists(requestParameters, 'personId')) {
+        if (requestParameters['personId'] == null) {
             throw new runtime.RequiredError(
                 'personId',
                 'Required parameter "personId" was null or undefined when calling list().'
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Club.ts b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Club.ts
index 513f4f1ea976..5e7c7f7f972f 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Club.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Club.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Owner } from './Owner';
 import {
     OwnerFromJSON,
@@ -46,21 +46,18 @@ export function ClubFromJSON(json: any): Club {
 }
 
 export function ClubFromJSONTyped(json: any, ignoreDiscriminator: boolean): Club {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'owner': !exists(json, 'owner') ? undefined : OwnerFromJSON(json['owner']),
+        'owner': json['owner'] == null ? undefined : OwnerFromJSON(json['owner']),
     };
 }
 
 export function ClubToJSON(value?: Club | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Owner.ts b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Owner.ts
index da532a1c5503..50e770a7d3f8 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Owner.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/models/Owner.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function OwnerFromJSON(json: any): Owner {
 }
 
 export function OwnerFromJSONTyped(json: any, ignoreDiscriminator: boolean): Owner {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function OwnerToJSON(value?: Owner | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/runtime.ts b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/runtime.ts
index 3daec13bee5b..191d8ea48935 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-nullable/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-nullable/runtime.ts
@@ -310,11 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/apis/DefaultApi.ts b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/apis/DefaultApi.ts
index fd8a011fc7cd..38da4f052ba3 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/apis/DefaultApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/apis/DefaultApi.ts
@@ -34,7 +34,7 @@ export class DefaultApi extends runtime.BaseAPI {
     /**
      */
     async listRaw(requestParameters: ListRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Club>> {
-        if (!runtime.exists(requestParameters, 'personId')) {
+        if (requestParameters['personId'] == null) {
             throw new runtime.RequiredError(
                 'personId',
                 'Required parameter "personId" was null or undefined when calling list().'
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Club.ts b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Club.ts
index 8ae80041ec65..58d15aca7e16 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Club.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Club.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Owner } from './Owner';
 import {
     OwnerFromJSON,
@@ -46,21 +46,18 @@ export function ClubFromJSON(json: any): Club {
 }
 
 export function ClubFromJSONTyped(json: any, ignoreDiscriminator: boolean): Club {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'owner': !exists(json, 'owner') ? undefined : OwnerFromJSON(json['owner']),
+        'owner': json['owner'] == null ? undefined : OwnerFromJSON(json['owner']),
     };
 }
 
 export function ClubToJSON(value?: Club | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Owner.ts b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Owner.ts
index da532a1c5503..50e770a7d3f8 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Owner.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/models/Owner.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function OwnerFromJSON(json: any): Owner {
 }
 
 export function OwnerFromJSONTyped(json: any, ignoreDiscriminator: boolean): Owner {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function OwnerToJSON(value?: Owner | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/runtime.ts b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/runtime.ts
index 3daec13bee5b..191d8ea48935 100644
--- a/samples/client/petstore/typescript-fetch/builds/allOf-readonly/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/allOf-readonly/runtime.ts
@@ -310,11 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/AnotherFakeApi.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/AnotherFakeApi.ts
index cec7e569d020..e63d48d42bd6 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/AnotherFakeApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/AnotherFakeApi.ts
@@ -36,7 +36,7 @@ export class AnotherFakeApi extends runtime.BaseAPI {
      * To test special tags
      */
     async _123testSpecialTagsRaw(requestParameters: 123testSpecialTagsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Client>> {
-        if (!runtime.exists(requestParameters, 'client')) {
+        if (requestParameters['client'] == null) {
             throw new runtime.RequiredError(
                 'client',
                 'Required parameter "client" was null or undefined when calling _123testSpecialTags().'
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/FakeApi.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/FakeApi.ts
index 12110a8f6a5b..484c15265f5f 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/FakeApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/FakeApi.ts
@@ -229,7 +229,7 @@ export class FakeApi extends runtime.BaseAPI {
      * test http signature authentication
      */
     async fakeHttpSignatureTestRaw(requestParameters: FakeHttpSignatureTestRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'pet')) {
+        if (requestParameters['pet'] == null) {
             throw new runtime.RequiredError(
                 'pet',
                 'Required parameter "pet" was null or undefined when calling fakeHttpSignatureTest().'
@@ -238,7 +238,7 @@ export class FakeApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'query1')) {
+        if (requestParameters['query1'] != null) {
             queryParameters['query_1'] = requestParameters['query1'];
         }
 
@@ -246,7 +246,7 @@ export class FakeApi extends runtime.BaseAPI {
 
         headerParameters['Content-Type'] = 'application/json';
 
-        if (runtime.exists(requestParameters, 'header1')) {
+        if (requestParameters['header1'] != null) {
             headerParameters['header_1'] = String(requestParameters['header1']);
         }
 
@@ -400,7 +400,7 @@ export class FakeApi extends runtime.BaseAPI {
      * Test serialization of enum (int) properties with examples
      */
     async fakePropertyEnumIntegerSerializeRaw(requestParameters: FakePropertyEnumIntegerSerializeRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<OuterObjectWithEnumProperty>> {
-        if (!runtime.exists(requestParameters, 'outerObjectWithEnumProperty')) {
+        if (requestParameters['outerObjectWithEnumProperty'] == null) {
             throw new runtime.RequiredError(
                 'outerObjectWithEnumProperty',
                 'Required parameter "outerObjectWithEnumProperty" was null or undefined when calling fakePropertyEnumIntegerSerialize().'
@@ -437,7 +437,7 @@ export class FakeApi extends runtime.BaseAPI {
      * test referenced additionalProperties
      */
     async testAdditionalPropertiesReferenceRaw(requestParameters: TestAdditionalPropertiesReferenceRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'requestBody')) {
+        if (requestParameters['requestBody'] == null) {
             throw new runtime.RequiredError(
                 'requestBody',
                 'Required parameter "requestBody" was null or undefined when calling testAdditionalPropertiesReference().'
@@ -473,7 +473,7 @@ export class FakeApi extends runtime.BaseAPI {
      * For this test, the body has to be a binary file.
      */
     async testBodyWithBinaryRaw(requestParameters: TestBodyWithBinaryRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling testBodyWithBinary().'
@@ -508,7 +508,7 @@ export class FakeApi extends runtime.BaseAPI {
      * For this test, the body for this request must reference a schema named `File`.
      */
     async testBodyWithFileSchemaRaw(requestParameters: TestBodyWithFileSchemaRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'fileSchemaTestClass')) {
+        if (requestParameters['fileSchemaTestClass'] == null) {
             throw new runtime.RequiredError(
                 'fileSchemaTestClass',
                 'Required parameter "fileSchemaTestClass" was null or undefined when calling testBodyWithFileSchema().'
@@ -542,14 +542,14 @@ export class FakeApi extends runtime.BaseAPI {
     /**
      */
     async testBodyWithQueryParamsRaw(requestParameters: TestBodyWithQueryParamsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'query')) {
+        if (requestParameters['query'] == null) {
             throw new runtime.RequiredError(
                 'query',
                 'Required parameter "query" was null or undefined when calling testBodyWithQueryParams().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'user')) {
+        if (requestParameters['user'] == null) {
             throw new runtime.RequiredError(
                 'user',
                 'Required parameter "user" was null or undefined when calling testBodyWithQueryParams().'
@@ -558,7 +558,7 @@ export class FakeApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'query')) {
+        if (requestParameters['query'] != null) {
             queryParameters['query'] = requestParameters['query'];
         }
 
@@ -588,7 +588,7 @@ export class FakeApi extends runtime.BaseAPI {
      * To test \"client\" model
      */
     async testClientModelRaw(requestParameters: TestClientModelRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Client>> {
-        if (!runtime.exists(requestParameters, 'client')) {
+        if (requestParameters['client'] == null) {
             throw new runtime.RequiredError(
                 'client',
                 'Required parameter "client" was null or undefined when calling testClientModel().'
@@ -626,28 +626,28 @@ export class FakeApi extends runtime.BaseAPI {
      * Fake endpoint for testing various parameters 假端點 偽のエンドポイント 가짜 엔드 포인트 
      */
     async testEndpointParametersRaw(requestParameters: TestEndpointParametersRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'number')) {
+        if (requestParameters['number'] == null) {
             throw new runtime.RequiredError(
                 'number',
                 'Required parameter "number" was null or undefined when calling testEndpointParameters().'
             );
         }
 
-        if (!runtime.exists(requestParameters, '_double')) {
+        if (requestParameters['_double'] == null) {
             throw new runtime.RequiredError(
                 '_double',
                 'Required parameter "_double" was null or undefined when calling testEndpointParameters().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'patternWithoutDelimiter')) {
+        if (requestParameters['patternWithoutDelimiter'] == null) {
             throw new runtime.RequiredError(
                 'patternWithoutDelimiter',
                 'Required parameter "patternWithoutDelimiter" was null or undefined when calling testEndpointParameters().'
             );
         }
 
-        if (!runtime.exists(requestParameters, '_byte')) {
+        if (requestParameters['_byte'] == null) {
             throw new runtime.RequiredError(
                 '_byte',
                 'Required parameter "_byte" was null or undefined when calling testEndpointParameters().'
@@ -677,59 +677,59 @@ export class FakeApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'integer')) {
+        if (requestParameters['integer'] != null) {
             formParams.append('integer', requestParameters['integer'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'int32')) {
+        if (requestParameters['int32'] != null) {
             formParams.append('int32', requestParameters['int32'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'int64')) {
+        if (requestParameters['int64'] != null) {
             formParams.append('int64', requestParameters['int64'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'number')) {
+        if (requestParameters['number'] != null) {
             formParams.append('number', requestParameters['number'] as any);
         }
 
-        if (runtime.exists(requestParameters, '_float')) {
+        if (requestParameters['_float'] != null) {
             formParams.append('float', requestParameters['_float'] as any);
         }
 
-        if (runtime.exists(requestParameters, '_double')) {
+        if (requestParameters['_double'] != null) {
             formParams.append('double', requestParameters['_double'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'string')) {
+        if (requestParameters['string'] != null) {
             formParams.append('string', requestParameters['string'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'patternWithoutDelimiter')) {
+        if (requestParameters['patternWithoutDelimiter'] != null) {
             formParams.append('pattern_without_delimiter', requestParameters['patternWithoutDelimiter'] as any);
         }
 
-        if (runtime.exists(requestParameters, '_byte')) {
+        if (requestParameters['_byte'] != null) {
             formParams.append('byte', requestParameters['_byte'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'binary')) {
+        if (requestParameters['binary'] != null) {
             formParams.append('binary', requestParameters['binary'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'date')) {
+        if (requestParameters['date'] != null) {
             formParams.append('date', requestParameters['date'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'dateTime')) {
+        if (requestParameters['dateTime'] != null) {
             formParams.append('dateTime', requestParameters['dateTime'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] != null) {
             formParams.append('password', requestParameters['password'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'callback')) {
+        if (requestParameters['callback'] != null) {
             formParams.append('callback', requestParameters['callback'] as any);
         }
 
@@ -759,33 +759,33 @@ export class FakeApi extends runtime.BaseAPI {
     async testEnumParametersRaw(requestParameters: TestEnumParametersRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'enumQueryStringArray')) {
+        if (requestParameters['enumQueryStringArray'] != null) {
             queryParameters['enum_query_string_array'] = requestParameters['enumQueryStringArray'];
         }
 
-        if (runtime.exists(requestParameters, 'enumQueryString')) {
+        if (requestParameters['enumQueryString'] != null) {
             queryParameters['enum_query_string'] = requestParameters['enumQueryString'];
         }
 
-        if (runtime.exists(requestParameters, 'enumQueryInteger')) {
+        if (requestParameters['enumQueryInteger'] != null) {
             queryParameters['enum_query_integer'] = requestParameters['enumQueryInteger'];
         }
 
-        if (runtime.exists(requestParameters, 'enumQueryDouble')) {
+        if (requestParameters['enumQueryDouble'] != null) {
             queryParameters['enum_query_double'] = requestParameters['enumQueryDouble'];
         }
 
-        if (runtime.exists(requestParameters, 'enumQueryModelArray')) {
+        if (requestParameters['enumQueryModelArray'] != null) {
             queryParameters['enum_query_model_array'] = requestParameters['enumQueryModelArray'];
         }
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'enumHeaderStringArray')) {
+        if (requestParameters['enumHeaderStringArray'] != null) {
             headerParameters['enum_header_string_array'] = requestParameters['enumHeaderStringArray']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
-        if (runtime.exists(requestParameters, 'enumHeaderString')) {
+        if (requestParameters['enumHeaderString'] != null) {
             headerParameters['enum_header_string'] = String(requestParameters['enumHeaderString']);
         }
 
@@ -803,11 +803,11 @@ export class FakeApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'enumFormStringArray')) {
+        if (requestParameters['enumFormStringArray'] != null) {
             formParams.append('enum_form_string_array', requestParameters['enumFormStringArray']!.join(runtime.COLLECTION_FORMATS["csv"]));
         }
 
-        if (runtime.exists(requestParameters, 'enumFormString')) {
+        if (requestParameters['enumFormString'] != null) {
             formParams.append('enum_form_string', requestParameters['enumFormString'] as any);
         }
 
@@ -835,21 +835,21 @@ export class FakeApi extends runtime.BaseAPI {
      * Fake endpoint to test group parameters (optional)
      */
     async testGroupParametersRaw(requestParameters: TestGroupParametersRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'requiredStringGroup')) {
+        if (requestParameters['requiredStringGroup'] == null) {
             throw new runtime.RequiredError(
                 'requiredStringGroup',
                 'Required parameter "requiredStringGroup" was null or undefined when calling testGroupParameters().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'requiredBooleanGroup')) {
+        if (requestParameters['requiredBooleanGroup'] == null) {
             throw new runtime.RequiredError(
                 'requiredBooleanGroup',
                 'Required parameter "requiredBooleanGroup" was null or undefined when calling testGroupParameters().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'requiredInt64Group')) {
+        if (requestParameters['requiredInt64Group'] == null) {
             throw new runtime.RequiredError(
                 'requiredInt64Group',
                 'Required parameter "requiredInt64Group" was null or undefined when calling testGroupParameters().'
@@ -858,29 +858,29 @@ export class FakeApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'requiredStringGroup')) {
+        if (requestParameters['requiredStringGroup'] != null) {
             queryParameters['required_string_group'] = requestParameters['requiredStringGroup'];
         }
 
-        if (runtime.exists(requestParameters, 'requiredInt64Group')) {
+        if (requestParameters['requiredInt64Group'] != null) {
             queryParameters['required_int64_group'] = requestParameters['requiredInt64Group'];
         }
 
-        if (runtime.exists(requestParameters, 'stringGroup')) {
+        if (requestParameters['stringGroup'] != null) {
             queryParameters['string_group'] = requestParameters['stringGroup'];
         }
 
-        if (runtime.exists(requestParameters, 'int64Group')) {
+        if (requestParameters['int64Group'] != null) {
             queryParameters['int64_group'] = requestParameters['int64Group'];
         }
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'requiredBooleanGroup')) {
+        if (requestParameters['requiredBooleanGroup'] != null) {
             headerParameters['required_boolean_group'] = String(requestParameters['requiredBooleanGroup']);
         }
 
-        if (runtime.exists(requestParameters, 'booleanGroup')) {
+        if (requestParameters['booleanGroup'] != null) {
             headerParameters['boolean_group'] = String(requestParameters['booleanGroup']);
         }
 
@@ -915,7 +915,7 @@ export class FakeApi extends runtime.BaseAPI {
      * test inline additionalProperties
      */
     async testInlineAdditionalPropertiesRaw(requestParameters: TestInlineAdditionalPropertiesRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'requestBody')) {
+        if (requestParameters['requestBody'] == null) {
             throw new runtime.RequiredError(
                 'requestBody',
                 'Required parameter "requestBody" was null or undefined when calling testInlineAdditionalProperties().'
@@ -952,7 +952,7 @@ export class FakeApi extends runtime.BaseAPI {
      * test inline free-form additionalProperties
      */
     async testInlineFreeformAdditionalPropertiesRaw(requestParameters: TestInlineFreeformAdditionalPropertiesOperationRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'testInlineFreeformAdditionalPropertiesRequest')) {
+        if (requestParameters['testInlineFreeformAdditionalPropertiesRequest'] == null) {
             throw new runtime.RequiredError(
                 'testInlineFreeformAdditionalPropertiesRequest',
                 'Required parameter "testInlineFreeformAdditionalPropertiesRequest" was null or undefined when calling testInlineFreeformAdditionalProperties().'
@@ -989,14 +989,14 @@ export class FakeApi extends runtime.BaseAPI {
      * test json serialization of form data
      */
     async testJsonFormDataRaw(requestParameters: TestJsonFormDataRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'param')) {
+        if (requestParameters['param'] == null) {
             throw new runtime.RequiredError(
                 'param',
                 'Required parameter "param" was null or undefined when calling testJsonFormData().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'param2')) {
+        if (requestParameters['param2'] == null) {
             throw new runtime.RequiredError(
                 'param2',
                 'Required parameter "param2" was null or undefined when calling testJsonFormData().'
@@ -1021,11 +1021,11 @@ export class FakeApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'param')) {
+        if (requestParameters['param'] != null) {
             formParams.append('param', requestParameters['param'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'param2')) {
+        if (requestParameters['param2'] != null) {
             formParams.append('param2', requestParameters['param2'] as any);
         }
 
@@ -1053,7 +1053,7 @@ export class FakeApi extends runtime.BaseAPI {
      * test nullable parent property
      */
     async testNullableRaw(requestParameters: TestNullableRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'childWithNullable')) {
+        if (requestParameters['childWithNullable'] == null) {
             throw new runtime.RequiredError(
                 'childWithNullable',
                 'Required parameter "childWithNullable" was null or undefined when calling testNullable().'
@@ -1089,42 +1089,42 @@ export class FakeApi extends runtime.BaseAPI {
      * To test the collection format in query parameters
      */
     async testQueryParameterCollectionFormatRaw(requestParameters: TestQueryParameterCollectionFormatRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'pipe')) {
+        if (requestParameters['pipe'] == null) {
             throw new runtime.RequiredError(
                 'pipe',
                 'Required parameter "pipe" was null or undefined when calling testQueryParameterCollectionFormat().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'ioutil')) {
+        if (requestParameters['ioutil'] == null) {
             throw new runtime.RequiredError(
                 'ioutil',
                 'Required parameter "ioutil" was null or undefined when calling testQueryParameterCollectionFormat().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'http')) {
+        if (requestParameters['http'] == null) {
             throw new runtime.RequiredError(
                 'http',
                 'Required parameter "http" was null or undefined when calling testQueryParameterCollectionFormat().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'url')) {
+        if (requestParameters['url'] == null) {
             throw new runtime.RequiredError(
                 'url',
                 'Required parameter "url" was null or undefined when calling testQueryParameterCollectionFormat().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'context')) {
+        if (requestParameters['context'] == null) {
             throw new runtime.RequiredError(
                 'context',
                 'Required parameter "context" was null or undefined when calling testQueryParameterCollectionFormat().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'allowEmpty')) {
+        if (requestParameters['allowEmpty'] == null) {
             throw new runtime.RequiredError(
                 'allowEmpty',
                 'Required parameter "allowEmpty" was null or undefined when calling testQueryParameterCollectionFormat().'
@@ -1133,31 +1133,31 @@ export class FakeApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'pipe')) {
+        if (requestParameters['pipe'] != null) {
             queryParameters['pipe'] = requestParameters['pipe']!.join(runtime.COLLECTION_FORMATS["pipes"]);
         }
 
-        if (runtime.exists(requestParameters, 'ioutil')) {
+        if (requestParameters['ioutil'] != null) {
             queryParameters['ioutil'] = requestParameters['ioutil']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
-        if (runtime.exists(requestParameters, 'http')) {
+        if (requestParameters['http'] != null) {
             queryParameters['http'] = requestParameters['http']!.join(runtime.COLLECTION_FORMATS["ssv"]);
         }
 
-        if (runtime.exists(requestParameters, 'url')) {
+        if (requestParameters['url'] != null) {
             queryParameters['url'] = requestParameters['url']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
-        if (runtime.exists(requestParameters, 'context')) {
+        if (requestParameters['context'] != null) {
             queryParameters['context'] = requestParameters['context'];
         }
 
-        if (runtime.exists(requestParameters, 'language')) {
+        if (requestParameters['language'] != null) {
             queryParameters['language'] = requestParameters['language'];
         }
 
-        if (runtime.exists(requestParameters, 'allowEmpty')) {
+        if (requestParameters['allowEmpty'] != null) {
             queryParameters['allowEmpty'] = requestParameters['allowEmpty'];
         }
 
@@ -1185,7 +1185,7 @@ export class FakeApi extends runtime.BaseAPI {
      * test referenced string map
      */
     async testStringMapReferenceRaw(requestParameters: TestStringMapReferenceRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'requestBody')) {
+        if (requestParameters['requestBody'] == null) {
             throw new runtime.RequiredError(
                 'requestBody',
                 'Required parameter "requestBody" was null or undefined when calling testStringMapReference().'
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/FakeClassnameTags123Api.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/FakeClassnameTags123Api.ts
index ce9f567a5324..e2cf17eb7a26 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/FakeClassnameTags123Api.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/FakeClassnameTags123Api.ts
@@ -36,7 +36,7 @@ export class FakeClassnameTags123Api extends runtime.BaseAPI {
      * To test class name in snake case
      */
     async testClassnameRaw(requestParameters: TestClassnameRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Client>> {
-        if (!runtime.exists(requestParameters, 'client')) {
+        if (requestParameters['client'] == null) {
             throw new runtime.RequiredError(
                 'client',
                 'Required parameter "client" was null or undefined when calling testClassname().'
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/PetApi.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/PetApi.ts
index 389f7d7077f3..5b77f2de4486 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/PetApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/PetApi.ts
@@ -78,7 +78,7 @@ export class PetApi extends runtime.BaseAPI {
      * Add a new pet to the store
      */
     async addPetRaw(requestParameters: AddPetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'pet')) {
+        if (requestParameters['pet'] == null) {
             throw new runtime.RequiredError(
                 'pet',
                 'Required parameter "pet" was null or undefined when calling addPet().'
@@ -120,7 +120,7 @@ export class PetApi extends runtime.BaseAPI {
      * Deletes a pet
      */
     async deletePetRaw(requestParameters: DeletePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling deletePet().'
@@ -131,7 +131,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'apiKey')) {
+        if (requestParameters['apiKey'] != null) {
             headerParameters['api_key'] = String(requestParameters['apiKey']);
         }
 
@@ -163,7 +163,7 @@ export class PetApi extends runtime.BaseAPI {
      * Finds Pets by status
      */
     async findPetsByStatusRaw(requestParameters: FindPetsByStatusRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] == null) {
             throw new runtime.RequiredError(
                 'status',
                 'Required parameter "status" was null or undefined when calling findPetsByStatus().'
@@ -172,7 +172,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             queryParameters['status'] = requestParameters['status']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -208,7 +208,7 @@ export class PetApi extends runtime.BaseAPI {
      * @deprecated
      */
     async findPetsByTagsRaw(requestParameters: FindPetsByTagsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Set<Pet>>> {
-        if (!runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] == null) {
             throw new runtime.RequiredError(
                 'tags',
                 'Required parameter "tags" was null or undefined when calling findPetsByTags().'
@@ -217,7 +217,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] != null) {
             queryParameters['tags'] = Array.from(requestParameters['tags'])!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -253,7 +253,7 @@ export class PetApi extends runtime.BaseAPI {
      * Find pet by ID
      */
     async getPetByIdRaw(requestParameters: GetPetByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Pet>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling getPetById().'
@@ -292,7 +292,7 @@ export class PetApi extends runtime.BaseAPI {
      * Update an existing pet
      */
     async updatePetRaw(requestParameters: UpdatePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'pet')) {
+        if (requestParameters['pet'] == null) {
             throw new runtime.RequiredError(
                 'pet',
                 'Required parameter "pet" was null or undefined when calling updatePet().'
@@ -334,7 +334,7 @@ export class PetApi extends runtime.BaseAPI {
      * Updates a pet in the store with form data
      */
     async updatePetWithFormRaw(requestParameters: UpdatePetWithFormRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling updatePetWithForm().'
@@ -364,11 +364,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'name')) {
+        if (requestParameters['name'] != null) {
             formParams.append('name', requestParameters['name'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             formParams.append('status', requestParameters['status'] as any);
         }
 
@@ -396,7 +396,7 @@ export class PetApi extends runtime.BaseAPI {
      * uploads an image
      */
     async uploadFileRaw(requestParameters: UploadFileRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<ModelApiResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling uploadFile().'
@@ -428,11 +428,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'additionalMetadata')) {
+        if (requestParameters['additionalMetadata'] != null) {
             formParams.append('additionalMetadata', requestParameters['additionalMetadata'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'file')) {
+        if (requestParameters['file'] != null) {
             formParams.append('file', requestParameters['file'] as any);
         }
 
@@ -461,14 +461,14 @@ export class PetApi extends runtime.BaseAPI {
      * uploads an image (required)
      */
     async uploadFileWithRequiredFileRaw(requestParameters: UploadFileWithRequiredFileRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<ModelApiResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling uploadFileWithRequiredFile().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'requiredFile')) {
+        if (requestParameters['requiredFile'] == null) {
             throw new runtime.RequiredError(
                 'requiredFile',
                 'Required parameter "requiredFile" was null or undefined when calling uploadFileWithRequiredFile().'
@@ -500,11 +500,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'additionalMetadata')) {
+        if (requestParameters['additionalMetadata'] != null) {
             formParams.append('additionalMetadata', requestParameters['additionalMetadata'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'requiredFile')) {
+        if (requestParameters['requiredFile'] != null) {
             formParams.append('requiredFile', requestParameters['requiredFile'] as any);
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/StoreApi.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/StoreApi.ts
index 4444f71d4cd0..4a7ee687d8dc 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/StoreApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/StoreApi.ts
@@ -44,7 +44,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Delete purchase order by ID
      */
     async deleteOrderRaw(requestParameters: DeleteOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling deleteOrder().'
@@ -110,7 +110,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Find purchase order by ID
      */
     async getOrderByIdRaw(requestParameters: GetOrderByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling getOrderById().'
@@ -145,7 +145,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Place an order for a pet
      */
     async placeOrderRaw(requestParameters: PlaceOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'order')) {
+        if (requestParameters['order'] == null) {
             throw new runtime.RequiredError(
                 'order',
                 'Required parameter "order" was null or undefined when calling placeOrder().'
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/UserApi.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/UserApi.ts
index 36114480012f..b151cb1eaaca 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/UserApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/apis/UserApi.ts
@@ -62,7 +62,7 @@ export class UserApi extends runtime.BaseAPI {
      * Create user
      */
     async createUserRaw(requestParameters: CreateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'user')) {
+        if (requestParameters['user'] == null) {
             throw new runtime.RequiredError(
                 'user',
                 'Required parameter "user" was null or undefined when calling createUser().'
@@ -99,7 +99,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithArrayInputRaw(requestParameters: CreateUsersWithArrayInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'user')) {
+        if (requestParameters['user'] == null) {
             throw new runtime.RequiredError(
                 'user',
                 'Required parameter "user" was null or undefined when calling createUsersWithArrayInput().'
@@ -136,7 +136,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithListInputRaw(requestParameters: CreateUsersWithListInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'user')) {
+        if (requestParameters['user'] == null) {
             throw new runtime.RequiredError(
                 'user',
                 'Required parameter "user" was null or undefined when calling createUsersWithListInput().'
@@ -173,7 +173,7 @@ export class UserApi extends runtime.BaseAPI {
      * Delete user
      */
     async deleteUserRaw(requestParameters: DeleteUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling deleteUser().'
@@ -207,7 +207,7 @@ export class UserApi extends runtime.BaseAPI {
      * Get user by user name
      */
     async getUserByNameRaw(requestParameters: GetUserByNameRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<User>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling getUserByName().'
@@ -242,14 +242,14 @@ export class UserApi extends runtime.BaseAPI {
      * Logs user into the system
      */
     async loginUserRaw(requestParameters: LoginUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<string>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling loginUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] == null) {
             throw new runtime.RequiredError(
                 'password',
                 'Required parameter "password" was null or undefined when calling loginUser().'
@@ -258,11 +258,11 @@ export class UserApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] != null) {
             queryParameters['username'] = requestParameters['username'];
         }
 
-        if (runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] != null) {
             queryParameters['password'] = requestParameters['password'];
         }
 
@@ -323,14 +323,14 @@ export class UserApi extends runtime.BaseAPI {
      * Updated user
      */
     async updateUserRaw(requestParameters: UpdateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling updateUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'user')) {
+        if (requestParameters['user'] == null) {
             throw new runtime.RequiredError(
                 'user',
                 'Required parameter "user" was null or undefined when calling updateUser().'
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AdditionalPropertiesClass.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AdditionalPropertiesClass.ts
index 429a86538cd1..f0c934a942c6 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AdditionalPropertiesClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AdditionalPropertiesClass.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -45,22 +45,19 @@ export function AdditionalPropertiesClassFromJSON(json: any): AdditionalProperti
 }
 
 export function AdditionalPropertiesClassFromJSONTyped(json: any, ignoreDiscriminator: boolean): AdditionalPropertiesClass {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'mapProperty': !exists(json, 'map_property') ? undefined : json['map_property'],
-        'mapOfMapProperty': !exists(json, 'map_of_map_property') ? undefined : json['map_of_map_property'],
+        'mapProperty': json['map_property'] == null ? undefined : json['map_property'],
+        'mapOfMapProperty': json['map_of_map_property'] == null ? undefined : json['map_of_map_property'],
     };
 }
 
 export function AdditionalPropertiesClassToJSON(value?: AdditionalPropertiesClass | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AllOfWithSingleRef.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AllOfWithSingleRef.ts
index 5fc70179d079..8f9802529e4b 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AllOfWithSingleRef.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/AllOfWithSingleRef.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { SingleRefType } from './SingleRefType';
 import {
     SingleRefTypeFromJSON,
@@ -52,22 +52,19 @@ export function AllOfWithSingleRefFromJSON(json: any): AllOfWithSingleRef {
 }
 
 export function AllOfWithSingleRefFromJSONTyped(json: any, ignoreDiscriminator: boolean): AllOfWithSingleRef {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'username': !exists(json, 'username') ? undefined : json['username'],
-        'singleRefType': !exists(json, 'SingleRefType') ? undefined : SingleRefTypeFromJSON(json['SingleRefType']),
+        'username': json['username'] == null ? undefined : json['username'],
+        'singleRefType': json['SingleRefType'] == null ? undefined : SingleRefTypeFromJSON(json['SingleRefType']),
     };
 }
 
 export function AllOfWithSingleRefToJSON(value?: AllOfWithSingleRef | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Animal.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Animal.ts
index 2d08a8e50e4a..a1b091662a1d 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Animal.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Animal.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import {
      CatFromJSONTyped,
      DogFromJSONTyped
@@ -51,7 +51,7 @@ export function AnimalFromJSON(json: any): Animal {
 }
 
 export function AnimalFromJSONTyped(json: any, ignoreDiscriminator: boolean): Animal {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     if (!ignoreDiscriminator) {
@@ -65,16 +65,13 @@ export function AnimalFromJSONTyped(json: any, ignoreDiscriminator: boolean): An
     return {
         
         'className': json['className'],
-        'color': !exists(json, 'color') ? undefined : json['color'],
+        'color': json['color'] == null ? undefined : json['color'],
     };
 }
 
 export function AnimalToJSON(value?: Animal | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfArrayOfNumberOnly.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfArrayOfNumberOnly.ts
index 6aca52358716..6e4c4ac139c3 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfArrayOfNumberOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfArrayOfNumberOnly.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function ArrayOfArrayOfNumberOnlyFromJSON(json: any): ArrayOfArrayOfNumbe
 }
 
 export function ArrayOfArrayOfNumberOnlyFromJSONTyped(json: any, ignoreDiscriminator: boolean): ArrayOfArrayOfNumberOnly {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'arrayArrayNumber': !exists(json, 'ArrayArrayNumber') ? undefined : json['ArrayArrayNumber'],
+        'arrayArrayNumber': json['ArrayArrayNumber'] == null ? undefined : json['ArrayArrayNumber'],
     };
 }
 
 export function ArrayOfArrayOfNumberOnlyToJSON(value?: ArrayOfArrayOfNumberOnly | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfNumberOnly.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfNumberOnly.ts
index b39a628ed121..4c0e2e98805b 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfNumberOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayOfNumberOnly.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function ArrayOfNumberOnlyFromJSON(json: any): ArrayOfNumberOnly {
 }
 
 export function ArrayOfNumberOnlyFromJSONTyped(json: any, ignoreDiscriminator: boolean): ArrayOfNumberOnly {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'arrayNumber': !exists(json, 'ArrayNumber') ? undefined : json['ArrayNumber'],
+        'arrayNumber': json['ArrayNumber'] == null ? undefined : json['ArrayNumber'],
     };
 }
 
 export function ArrayOfNumberOnlyToJSON(value?: ArrayOfNumberOnly | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayTest.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayTest.ts
index 2396708971fa..2cdb8f33fa6f 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ArrayTest.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { ReadOnlyFirst } from './ReadOnlyFirst';
 import {
     ReadOnlyFirstFromJSON,
@@ -58,23 +58,20 @@ export function ArrayTestFromJSON(json: any): ArrayTest {
 }
 
 export function ArrayTestFromJSONTyped(json: any, ignoreDiscriminator: boolean): ArrayTest {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'arrayOfString': !exists(json, 'array_of_string') ? undefined : json['array_of_string'],
-        'arrayArrayOfInteger': !exists(json, 'array_array_of_integer') ? undefined : json['array_array_of_integer'],
-        'arrayArrayOfModel': !exists(json, 'array_array_of_model') ? undefined : json['array_array_of_model'],
+        'arrayOfString': json['array_of_string'] == null ? undefined : json['array_of_string'],
+        'arrayArrayOfInteger': json['array_array_of_integer'] == null ? undefined : json['array_array_of_integer'],
+        'arrayArrayOfModel': json['array_array_of_model'] == null ? undefined : json['array_array_of_model'],
     };
 }
 
 export function ArrayTestToJSON(value?: ArrayTest | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Capitalization.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Capitalization.ts
index 30a6a48dfde4..1d7e59d6099d 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Capitalization.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Capitalization.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -70,26 +70,23 @@ export function CapitalizationFromJSON(json: any): Capitalization {
 }
 
 export function CapitalizationFromJSONTyped(json: any, ignoreDiscriminator: boolean): Capitalization {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'smallCamel': !exists(json, 'smallCamel') ? undefined : json['smallCamel'],
-        'capitalCamel': !exists(json, 'CapitalCamel') ? undefined : json['CapitalCamel'],
-        'smallSnake': !exists(json, 'small_Snake') ? undefined : json['small_Snake'],
-        'capitalSnake': !exists(json, 'Capital_Snake') ? undefined : json['Capital_Snake'],
-        'sCAETHFlowPoints': !exists(json, 'SCA_ETH_Flow_Points') ? undefined : json['SCA_ETH_Flow_Points'],
-        'aTTNAME': !exists(json, 'ATT_NAME') ? undefined : json['ATT_NAME'],
+        'smallCamel': json['smallCamel'] == null ? undefined : json['smallCamel'],
+        'capitalCamel': json['CapitalCamel'] == null ? undefined : json['CapitalCamel'],
+        'smallSnake': json['small_Snake'] == null ? undefined : json['small_Snake'],
+        'capitalSnake': json['Capital_Snake'] == null ? undefined : json['Capital_Snake'],
+        'sCAETHFlowPoints': json['SCA_ETH_Flow_Points'] == null ? undefined : json['SCA_ETH_Flow_Points'],
+        'aTTNAME': json['ATT_NAME'] == null ? undefined : json['ATT_NAME'],
     };
 }
 
 export function CapitalizationToJSON(value?: Capitalization | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Cat.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Cat.ts
index 94507ba96b1f..3bc2bd8cdb13 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Cat.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Cat.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Animal } from './Animal';
 import {
     AnimalFromJSON,
@@ -46,21 +46,18 @@ export function CatFromJSON(json: any): Cat {
 }
 
 export function CatFromJSONTyped(json: any, ignoreDiscriminator: boolean): Cat {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         ...AnimalFromJSONTyped(json, ignoreDiscriminator),
-        'declawed': !exists(json, 'declawed') ? undefined : json['declawed'],
+        'declawed': json['declawed'] == null ? undefined : json['declawed'],
     };
 }
 
 export function CatToJSON(value?: Cat | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         ...AnimalToJSON(value),
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Category.ts
index 3b4d927fb025..e571880c8e1e 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Category.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -46,22 +46,19 @@ export function CategoryFromJSON(json: any): Category {
 }
 
 export function CategoryFromJSONTyped(json: any, ignoreDiscriminator: boolean): Category {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
+        'id': json['id'] == null ? undefined : json['id'],
         'name': json['name'],
     };
 }
 
 export function CategoryToJSON(value?: Category | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ChildWithNullable.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ChildWithNullable.ts
index d1424e2d3d66..2ed247a45410 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ChildWithNullable.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ChildWithNullable.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { ParentWithNullable } from './ParentWithNullable';
 import {
     ParentWithNullableFromJSON,
@@ -48,21 +48,18 @@ export function ChildWithNullableFromJSON(json: any): ChildWithNullable {
 }
 
 export function ChildWithNullableFromJSONTyped(json: any, ignoreDiscriminator: boolean): ChildWithNullable {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         ...ParentWithNullableFromJSONTyped(json, ignoreDiscriminator),
-        'otherProperty': !exists(json, 'otherProperty') ? undefined : json['otherProperty'],
+        'otherProperty': json['otherProperty'] == null ? undefined : json['otherProperty'],
     };
 }
 
 export function ChildWithNullableToJSON(value?: ChildWithNullable | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         ...ParentWithNullableToJSON(value),
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ClassModel.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ClassModel.ts
index 6309f0dfce81..6d9ad88c1c55 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ClassModel.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ClassModel.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Model for testing model with "_class" property
  * @export
@@ -39,21 +39,18 @@ export function ClassModelFromJSON(json: any): ClassModel {
 }
 
 export function ClassModelFromJSONTyped(json: any, ignoreDiscriminator: boolean): ClassModel {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        '_class': !exists(json, '_class') ? undefined : json['_class'],
+        '_class': json['_class'] == null ? undefined : json['_class'],
     };
 }
 
 export function ClassModelToJSON(value?: ClassModel | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Client.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Client.ts
index 96e2993ca963..807cd0777308 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Client.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Client.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function ClientFromJSON(json: any): Client {
 }
 
 export function ClientFromJSONTyped(json: any, ignoreDiscriminator: boolean): Client {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'client': !exists(json, 'client') ? undefined : json['client'],
+        'client': json['client'] == null ? undefined : json['client'],
     };
 }
 
 export function ClientToJSON(value?: Client | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/DeprecatedObject.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/DeprecatedObject.ts
index c8113b4c9a39..8eab37673066 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/DeprecatedObject.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/DeprecatedObject.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function DeprecatedObjectFromJSON(json: any): DeprecatedObject {
 }
 
 export function DeprecatedObjectFromJSONTyped(json: any, ignoreDiscriminator: boolean): DeprecatedObject {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function DeprecatedObjectToJSON(value?: DeprecatedObject | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Dog.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Dog.ts
index fec18e3e484c..921e17cb803f 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Dog.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Dog.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Animal } from './Animal';
 import {
     AnimalFromJSON,
@@ -46,21 +46,18 @@ export function DogFromJSON(json: any): Dog {
 }
 
 export function DogFromJSONTyped(json: any, ignoreDiscriminator: boolean): Dog {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         ...AnimalFromJSONTyped(json, ignoreDiscriminator),
-        'breed': !exists(json, 'breed') ? undefined : json['breed'],
+        'breed': json['breed'] == null ? undefined : json['breed'],
     };
 }
 
 export function DogToJSON(value?: Dog | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         ...AnimalToJSON(value),
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumArrays.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumArrays.ts
index 0c93a179655a..aa7200c92542 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumArrays.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumArrays.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -65,22 +65,19 @@ export function EnumArraysFromJSON(json: any): EnumArrays {
 }
 
 export function EnumArraysFromJSONTyped(json: any, ignoreDiscriminator: boolean): EnumArrays {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'justSymbol': !exists(json, 'just_symbol') ? undefined : json['just_symbol'],
-        'arrayEnum': !exists(json, 'array_enum') ? undefined : json['array_enum'],
+        'justSymbol': json['just_symbol'] == null ? undefined : json['just_symbol'],
+        'arrayEnum': json['array_enum'] == null ? undefined : json['array_enum'],
     };
 }
 
 export function EnumArraysToJSON(value?: EnumArrays | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumTest.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumTest.ts
index 9a8b5051c321..dbe67a7cf1f7 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/EnumTest.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { OuterEnum } from './OuterEnum';
 import {
     OuterEnumFromJSON,
@@ -147,28 +147,25 @@ export function EnumTestFromJSON(json: any): EnumTest {
 }
 
 export function EnumTestFromJSONTyped(json: any, ignoreDiscriminator: boolean): EnumTest {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'enumString': !exists(json, 'enum_string') ? undefined : json['enum_string'],
+        'enumString': json['enum_string'] == null ? undefined : json['enum_string'],
         'enumStringRequired': json['enum_string_required'],
-        'enumInteger': !exists(json, 'enum_integer') ? undefined : json['enum_integer'],
-        'enumNumber': !exists(json, 'enum_number') ? undefined : json['enum_number'],
-        'outerEnum': !exists(json, 'outerEnum') ? undefined : OuterEnumFromJSON(json['outerEnum']),
-        'outerEnumInteger': !exists(json, 'outerEnumInteger') ? undefined : OuterEnumIntegerFromJSON(json['outerEnumInteger']),
-        'outerEnumDefaultValue': !exists(json, 'outerEnumDefaultValue') ? undefined : OuterEnumDefaultValueFromJSON(json['outerEnumDefaultValue']),
-        'outerEnumIntegerDefaultValue': !exists(json, 'outerEnumIntegerDefaultValue') ? undefined : OuterEnumIntegerDefaultValueFromJSON(json['outerEnumIntegerDefaultValue']),
+        'enumInteger': json['enum_integer'] == null ? undefined : json['enum_integer'],
+        'enumNumber': json['enum_number'] == null ? undefined : json['enum_number'],
+        'outerEnum': json['outerEnum'] == null ? undefined : OuterEnumFromJSON(json['outerEnum']),
+        'outerEnumInteger': json['outerEnumInteger'] == null ? undefined : OuterEnumIntegerFromJSON(json['outerEnumInteger']),
+        'outerEnumDefaultValue': json['outerEnumDefaultValue'] == null ? undefined : OuterEnumDefaultValueFromJSON(json['outerEnumDefaultValue']),
+        'outerEnumIntegerDefaultValue': json['outerEnumIntegerDefaultValue'] == null ? undefined : OuterEnumIntegerDefaultValueFromJSON(json['outerEnumIntegerDefaultValue']),
     };
 }
 
 export function EnumTestToJSON(value?: EnumTest | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FakeBigDecimalMap200Response.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FakeBigDecimalMap200Response.ts
index bf4371a93c1e..732ed05b62a0 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FakeBigDecimalMap200Response.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FakeBigDecimalMap200Response.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -45,22 +45,19 @@ export function FakeBigDecimalMap200ResponseFromJSON(json: any): FakeBigDecimalM
 }
 
 export function FakeBigDecimalMap200ResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): FakeBigDecimalMap200Response {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'someId': !exists(json, 'someId') ? undefined : json['someId'],
-        'someMap': !exists(json, 'someMap') ? undefined : json['someMap'],
+        'someId': json['someId'] == null ? undefined : json['someId'],
+        'someMap': json['someMap'] == null ? undefined : json['someMap'],
     };
 }
 
 export function FakeBigDecimalMap200ResponseToJSON(value?: FakeBigDecimalMap200Response | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FileSchemaTestClass.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FileSchemaTestClass.ts
index 10e7e77f8f87..25bf3ba4dbf1 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FileSchemaTestClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FileSchemaTestClass.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -45,22 +45,19 @@ export function FileSchemaTestClassFromJSON(json: any): FileSchemaTestClass {
 }
 
 export function FileSchemaTestClassFromJSONTyped(json: any, ignoreDiscriminator: boolean): FileSchemaTestClass {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'file': !exists(json, 'file') ? undefined : json['file'],
-        'files': !exists(json, 'files') ? undefined : json['files'],
+        'file': json['file'] == null ? undefined : json['file'],
+        'files': json['files'] == null ? undefined : json['files'],
     };
 }
 
 export function FileSchemaTestClassToJSON(value?: FileSchemaTestClass | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Foo.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Foo.ts
index ec179ce581b5..feb5e47da049 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Foo.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Foo.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function FooFromJSON(json: any): Foo {
 }
 
 export function FooFromJSONTyped(json: any, ignoreDiscriminator: boolean): Foo {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'bar': !exists(json, 'bar') ? undefined : json['bar'],
+        'bar': json['bar'] == null ? undefined : json['bar'],
     };
 }
 
 export function FooToJSON(value?: Foo | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FooGetDefaultResponse.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FooGetDefaultResponse.ts
index ffd3e185c760..b834ab550db6 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FooGetDefaultResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FooGetDefaultResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Foo } from './Foo';
 import {
     FooFromJSON,
@@ -46,21 +46,18 @@ export function FooGetDefaultResponseFromJSON(json: any): FooGetDefaultResponse
 }
 
 export function FooGetDefaultResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): FooGetDefaultResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'string': !exists(json, 'string') ? undefined : FooFromJSON(json['string']),
+        'string': json['string'] == null ? undefined : FooFromJSON(json['string']),
     };
 }
 
 export function FooGetDefaultResponseToJSON(value?: FooGetDefaultResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FormatTest.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FormatTest.ts
index b509f3575a2f..f2113709741f 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FormatTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/FormatTest.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Decimal } from './Decimal';
 import {
     DecimalFromJSON,
@@ -140,36 +140,33 @@ export function FormatTestFromJSON(json: any): FormatTest {
 }
 
 export function FormatTestFromJSONTyped(json: any, ignoreDiscriminator: boolean): FormatTest {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'integer': !exists(json, 'integer') ? undefined : json['integer'],
-        'int32': !exists(json, 'int32') ? undefined : json['int32'],
-        'int64': !exists(json, 'int64') ? undefined : json['int64'],
+        'integer': json['integer'] == null ? undefined : json['integer'],
+        'int32': json['int32'] == null ? undefined : json['int32'],
+        'int64': json['int64'] == null ? undefined : json['int64'],
         'number': json['number'],
-        '_float': !exists(json, 'float') ? undefined : json['float'],
-        '_double': !exists(json, 'double') ? undefined : json['double'],
-        'decimal': !exists(json, 'decimal') ? undefined : DecimalFromJSON(json['decimal']),
-        'string': !exists(json, 'string') ? undefined : json['string'],
+        '_float': json['float'] == null ? undefined : json['float'],
+        '_double': json['double'] == null ? undefined : json['double'],
+        'decimal': json['decimal'] == null ? undefined : DecimalFromJSON(json['decimal']),
+        'string': json['string'] == null ? undefined : json['string'],
         '_byte': json['byte'],
-        'binary': !exists(json, 'binary') ? undefined : json['binary'],
+        'binary': json['binary'] == null ? undefined : json['binary'],
         'date': (new Date(json['date'])),
-        'dateTime': !exists(json, 'dateTime') ? undefined : (new Date(json['dateTime'])),
-        'uuid': !exists(json, 'uuid') ? undefined : json['uuid'],
+        'dateTime': json['dateTime'] == null ? undefined : (new Date(json['dateTime'])),
+        'uuid': json['uuid'] == null ? undefined : json['uuid'],
         'password': json['password'],
-        'patternWithDigits': !exists(json, 'pattern_with_digits') ? undefined : json['pattern_with_digits'],
-        'patternWithDigitsAndDelimiter': !exists(json, 'pattern_with_digits_and_delimiter') ? undefined : json['pattern_with_digits_and_delimiter'],
+        'patternWithDigits': json['pattern_with_digits'] == null ? undefined : json['pattern_with_digits'],
+        'patternWithDigitsAndDelimiter': json['pattern_with_digits_and_delimiter'] == null ? undefined : json['pattern_with_digits_and_delimiter'],
     };
 }
 
 export function FormatTestToJSON(value?: FormatTest | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
@@ -184,7 +181,7 @@ export function FormatTestToJSON(value?: FormatTest | null): any {
         'byte': value['_byte'],
         'binary': value['binary'],
         'date': ((value['date']).toISOString().substring(0,10)),
-        'dateTime': !exists(value, 'dateTime') ? undefined : ((value['dateTime']).toISOString()),
+        'dateTime': value['dateTime'] == null ? undefined : ((value['dateTime']).toISOString()),
         'uuid': value['uuid'],
         'password': value['password'],
         'pattern_with_digits': value['patternWithDigits'],
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HasOnlyReadOnly.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HasOnlyReadOnly.ts
index e59842cd3e2f..9cc2da8afdaa 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HasOnlyReadOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HasOnlyReadOnly.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -45,22 +45,19 @@ export function HasOnlyReadOnlyFromJSON(json: any): HasOnlyReadOnly {
 }
 
 export function HasOnlyReadOnlyFromJSONTyped(json: any, ignoreDiscriminator: boolean): HasOnlyReadOnly {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'bar': !exists(json, 'bar') ? undefined : json['bar'],
-        'foo': !exists(json, 'foo') ? undefined : json['foo'],
+        'bar': json['bar'] == null ? undefined : json['bar'],
+        'foo': json['foo'] == null ? undefined : json['foo'],
     };
 }
 
 export function HasOnlyReadOnlyToJSON(value?: HasOnlyReadOnly | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HealthCheckResult.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HealthCheckResult.ts
index 6d58f178cdcb..83a0cf52ee15 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HealthCheckResult.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/HealthCheckResult.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Just a string to inform instance is up and running. Make it nullable in hope to get it as pointer in generated model.
  * @export
@@ -39,21 +39,18 @@ export function HealthCheckResultFromJSON(json: any): HealthCheckResult {
 }
 
 export function HealthCheckResultFromJSONTyped(json: any, ignoreDiscriminator: boolean): HealthCheckResult {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'nullableMessage': !exists(json, 'NullableMessage') ? undefined : json['NullableMessage'],
+        'nullableMessage': json['NullableMessage'] == null ? undefined : json['NullableMessage'],
     };
 }
 
 export function HealthCheckResultToJSON(value?: HealthCheckResult | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/List.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/List.ts
index c4afb61a4de4..5513deda0790 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/List.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/List.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function ListFromJSON(json: any): List {
 }
 
 export function ListFromJSONTyped(json: any, ignoreDiscriminator: boolean): List {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        '_123list': !exists(json, '123-list') ? undefined : json['123-list'],
+        '_123list': json['123-list'] == null ? undefined : json['123-list'],
     };
 }
 
 export function ListToJSON(value?: List | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MapTest.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MapTest.ts
index b36dfda98d88..0ba03f9a901b 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MapTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MapTest.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -68,24 +68,21 @@ export function MapTestFromJSON(json: any): MapTest {
 }
 
 export function MapTestFromJSONTyped(json: any, ignoreDiscriminator: boolean): MapTest {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'mapMapOfString': !exists(json, 'map_map_of_string') ? undefined : json['map_map_of_string'],
-        'mapOfEnumString': !exists(json, 'map_of_enum_string') ? undefined : json['map_of_enum_string'],
-        'directMap': !exists(json, 'direct_map') ? undefined : json['direct_map'],
-        'indirectMap': !exists(json, 'indirect_map') ? undefined : json['indirect_map'],
+        'mapMapOfString': json['map_map_of_string'] == null ? undefined : json['map_map_of_string'],
+        'mapOfEnumString': json['map_of_enum_string'] == null ? undefined : json['map_of_enum_string'],
+        'directMap': json['direct_map'] == null ? undefined : json['direct_map'],
+        'indirectMap': json['indirect_map'] == null ? undefined : json['indirect_map'],
     };
 }
 
 export function MapTestToJSON(value?: MapTest | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MixedPropertiesAndAdditionalPropertiesClass.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MixedPropertiesAndAdditionalPropertiesClass.ts
index f22b2b72a221..156ccc8a342d 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MixedPropertiesAndAdditionalPropertiesClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MixedPropertiesAndAdditionalPropertiesClass.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Animal } from './Animal';
 import {
     AnimalFromJSON,
@@ -58,29 +58,26 @@ export function MixedPropertiesAndAdditionalPropertiesClassFromJSON(json: any):
 }
 
 export function MixedPropertiesAndAdditionalPropertiesClassFromJSONTyped(json: any, ignoreDiscriminator: boolean): MixedPropertiesAndAdditionalPropertiesClass {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'uuid': !exists(json, 'uuid') ? undefined : json['uuid'],
-        'dateTime': !exists(json, 'dateTime') ? undefined : (new Date(json['dateTime'])),
-        'map': !exists(json, 'map') ? undefined : (mapValues(json['map'], AnimalFromJSON)),
+        'uuid': json['uuid'] == null ? undefined : json['uuid'],
+        'dateTime': json['dateTime'] == null ? undefined : (new Date(json['dateTime'])),
+        'map': json['map'] == null ? undefined : (mapValues(json['map'], AnimalFromJSON)),
     };
 }
 
 export function MixedPropertiesAndAdditionalPropertiesClassToJSON(value?: MixedPropertiesAndAdditionalPropertiesClass | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
         'uuid': value['uuid'],
-        'dateTime': !exists(value, 'dateTime') ? undefined : ((value['dateTime']).toISOString()),
-        'map': !exists(value, 'map') ? undefined : (mapValues(value['map'], AnimalToJSON)),
+        'dateTime': value['dateTime'] == null ? undefined : ((value['dateTime']).toISOString()),
+        'map': value['map'] == null ? undefined : (mapValues(value['map'], AnimalToJSON)),
     };
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Model200Response.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Model200Response.ts
index 8cabcd2d4b3e..22570495bd69 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Model200Response.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Model200Response.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Model for testing model name starting with number
  * @export
@@ -45,22 +45,19 @@ export function Model200ResponseFromJSON(json: any): Model200Response {
 }
 
 export function Model200ResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): Model200Response {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'name': !exists(json, 'name') ? undefined : json['name'],
-        '_class': !exists(json, 'class') ? undefined : json['class'],
+        'name': json['name'] == null ? undefined : json['name'],
+        '_class': json['class'] == null ? undefined : json['class'],
     };
 }
 
 export function Model200ResponseToJSON(value?: Model200Response | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelApiResponse.ts
index b3685aea30ad..7116a59f8d22 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelApiResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -51,23 +51,20 @@ export function ModelApiResponseFromJSON(json: any): ModelApiResponse {
 }
 
 export function ModelApiResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): ModelApiResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'code': !exists(json, 'code') ? undefined : json['code'],
-        'type': !exists(json, 'type') ? undefined : json['type'],
-        'message': !exists(json, 'message') ? undefined : json['message'],
+        'code': json['code'] == null ? undefined : json['code'],
+        'type': json['type'] == null ? undefined : json['type'],
+        'message': json['message'] == null ? undefined : json['message'],
     };
 }
 
 export function ModelApiResponseToJSON(value?: ModelApiResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelFile.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelFile.ts
index 0611bad737d6..dc08a2f1474a 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelFile.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ModelFile.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Must be named `File` for test.
  * @export
@@ -39,21 +39,18 @@ export function ModelFileFromJSON(json: any): ModelFile {
 }
 
 export function ModelFileFromJSONTyped(json: any, ignoreDiscriminator: boolean): ModelFile {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'sourceURI': !exists(json, 'sourceURI') ? undefined : json['sourceURI'],
+        'sourceURI': json['sourceURI'] == null ? undefined : json['sourceURI'],
     };
 }
 
 export function ModelFileToJSON(value?: ModelFile | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Name.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Name.ts
index 4ba736c85a45..53ca17e6317f 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Name.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Name.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Model for testing model name same as property name
  * @export
@@ -58,24 +58,21 @@ export function NameFromJSON(json: any): Name {
 }
 
 export function NameFromJSONTyped(json: any, ignoreDiscriminator: boolean): Name {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
         'name': json['name'],
-        'snakeCase': !exists(json, 'snake_case') ? undefined : json['snake_case'],
-        'property': !exists(json, 'property') ? undefined : json['property'],
-        '_123number': !exists(json, '123Number') ? undefined : json['123Number'],
+        'snakeCase': json['snake_case'] == null ? undefined : json['snake_case'],
+        'property': json['property'] == null ? undefined : json['property'],
+        '_123number': json['123Number'] == null ? undefined : json['123Number'],
     };
 }
 
 export function NameToJSON(value?: Name | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NullableClass.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NullableClass.ts
index de25ab3382a4..b64cd7d8f452 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NullableClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NullableClass.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -106,33 +106,30 @@ export function NullableClassFromJSON(json: any): NullableClass {
 }
 
 export function NullableClassFromJSONTyped(json: any, ignoreDiscriminator: boolean): NullableClass {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
             ...json,
-        'integerProp': !exists(json, 'integer_prop') ? undefined : json['integer_prop'],
-        'numberProp': !exists(json, 'number_prop') ? undefined : json['number_prop'],
-        'booleanProp': !exists(json, 'boolean_prop') ? undefined : json['boolean_prop'],
-        'stringProp': !exists(json, 'string_prop') ? undefined : json['string_prop'],
-        'dateProp': !exists(json, 'date_prop') ? undefined : (new Date(json['date_prop'])),
-        'datetimeProp': !exists(json, 'datetime_prop') ? undefined : (new Date(json['datetime_prop'])),
-        'arrayNullableProp': !exists(json, 'array_nullable_prop') ? undefined : json['array_nullable_prop'],
-        'arrayAndItemsNullableProp': !exists(json, 'array_and_items_nullable_prop') ? undefined : json['array_and_items_nullable_prop'],
-        'arrayItemsNullable': !exists(json, 'array_items_nullable') ? undefined : json['array_items_nullable'],
-        'objectNullableProp': !exists(json, 'object_nullable_prop') ? undefined : json['object_nullable_prop'],
-        'objectAndItemsNullableProp': !exists(json, 'object_and_items_nullable_prop') ? undefined : json['object_and_items_nullable_prop'],
-        'objectItemsNullable': !exists(json, 'object_items_nullable') ? undefined : json['object_items_nullable'],
+        'integerProp': json['integer_prop'] == null ? undefined : json['integer_prop'],
+        'numberProp': json['number_prop'] == null ? undefined : json['number_prop'],
+        'booleanProp': json['boolean_prop'] == null ? undefined : json['boolean_prop'],
+        'stringProp': json['string_prop'] == null ? undefined : json['string_prop'],
+        'dateProp': json['date_prop'] == null ? undefined : (new Date(json['date_prop'])),
+        'datetimeProp': json['datetime_prop'] == null ? undefined : (new Date(json['datetime_prop'])),
+        'arrayNullableProp': json['array_nullable_prop'] == null ? undefined : json['array_nullable_prop'],
+        'arrayAndItemsNullableProp': json['array_and_items_nullable_prop'] == null ? undefined : json['array_and_items_nullable_prop'],
+        'arrayItemsNullable': json['array_items_nullable'] == null ? undefined : json['array_items_nullable'],
+        'objectNullableProp': json['object_nullable_prop'] == null ? undefined : json['object_nullable_prop'],
+        'objectAndItemsNullableProp': json['object_and_items_nullable_prop'] == null ? undefined : json['object_and_items_nullable_prop'],
+        'objectItemsNullable': json['object_items_nullable'] == null ? undefined : json['object_items_nullable'],
     };
 }
 
 export function NullableClassToJSON(value?: NullableClass | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
@@ -141,8 +138,8 @@ export function NullableClassToJSON(value?: NullableClass | null): any {
         'number_prop': value['numberProp'],
         'boolean_prop': value['booleanProp'],
         'string_prop': value['stringProp'],
-        'date_prop': !exists(value, 'dateProp') ? undefined : ((value['dateProp'] as any).toISOString().substring(0,10)),
-        'datetime_prop': !exists(value, 'datetimeProp') ? undefined : ((value['datetimeProp'] as any).toISOString()),
+        'date_prop': value['dateProp'] == null ? undefined : ((value['dateProp'] as any).toISOString().substring(0,10)),
+        'datetime_prop': value['datetimeProp'] == null ? undefined : ((value['datetimeProp'] as any).toISOString()),
         'array_nullable_prop': value['arrayNullableProp'],
         'array_and_items_nullable_prop': value['arrayAndItemsNullableProp'],
         'array_items_nullable': value['arrayItemsNullable'],
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NumberOnly.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NumberOnly.ts
index 7a25240d3537..155afb118e43 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NumberOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NumberOnly.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function NumberOnlyFromJSON(json: any): NumberOnly {
 }
 
 export function NumberOnlyFromJSONTyped(json: any, ignoreDiscriminator: boolean): NumberOnly {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'justNumber': !exists(json, 'JustNumber') ? undefined : json['JustNumber'],
+        'justNumber': json['JustNumber'] == null ? undefined : json['JustNumber'],
     };
 }
 
 export function NumberOnlyToJSON(value?: NumberOnly | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ObjectWithDeprecatedFields.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ObjectWithDeprecatedFields.ts
index 4a921582669f..d24ea8fc197e 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ObjectWithDeprecatedFields.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ObjectWithDeprecatedFields.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { DeprecatedObject } from './DeprecatedObject';
 import {
     DeprecatedObjectFromJSON,
@@ -67,24 +67,21 @@ export function ObjectWithDeprecatedFieldsFromJSON(json: any): ObjectWithDepreca
 }
 
 export function ObjectWithDeprecatedFieldsFromJSONTyped(json: any, ignoreDiscriminator: boolean): ObjectWithDeprecatedFields {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'uuid': !exists(json, 'uuid') ? undefined : json['uuid'],
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'deprecatedRef': !exists(json, 'deprecatedRef') ? undefined : DeprecatedObjectFromJSON(json['deprecatedRef']),
-        'bars': !exists(json, 'bars') ? undefined : json['bars'],
+        'uuid': json['uuid'] == null ? undefined : json['uuid'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'deprecatedRef': json['deprecatedRef'] == null ? undefined : DeprecatedObjectFromJSON(json['deprecatedRef']),
+        'bars': json['bars'] == null ? undefined : json['bars'],
     };
 }
 
 export function ObjectWithDeprecatedFieldsToJSON(value?: ObjectWithDeprecatedFields | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Order.ts
index 6f2b79b5c85b..8921894971d6 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Order.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -81,33 +81,30 @@ export function OrderFromJSON(json: any): Order {
 }
 
 export function OrderFromJSONTyped(json: any, ignoreDiscriminator: boolean): Order {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'petId': !exists(json, 'petId') ? undefined : json['petId'],
-        'quantity': !exists(json, 'quantity') ? undefined : json['quantity'],
-        'shipDate': !exists(json, 'shipDate') ? undefined : (new Date(json['shipDate'])),
-        'status': !exists(json, 'status') ? undefined : json['status'],
-        'complete': !exists(json, 'complete') ? undefined : json['complete'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'petId': json['petId'] == null ? undefined : json['petId'],
+        'quantity': json['quantity'] == null ? undefined : json['quantity'],
+        'shipDate': json['shipDate'] == null ? undefined : (new Date(json['shipDate'])),
+        'status': json['status'] == null ? undefined : json['status'],
+        'complete': json['complete'] == null ? undefined : json['complete'],
     };
 }
 
 export function OrderToJSON(value?: Order | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': !exists(value, 'shipDate') ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
         'status': value['status'],
         'complete': value['complete'],
     };
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterComposite.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterComposite.ts
index 14b4e4542fab..49d8ad5cc5b1 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterComposite.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterComposite.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -51,23 +51,20 @@ export function OuterCompositeFromJSON(json: any): OuterComposite {
 }
 
 export function OuterCompositeFromJSONTyped(json: any, ignoreDiscriminator: boolean): OuterComposite {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'myNumber': !exists(json, 'my_number') ? undefined : json['my_number'],
-        'myString': !exists(json, 'my_string') ? undefined : json['my_string'],
-        'myBoolean': !exists(json, 'my_boolean') ? undefined : json['my_boolean'],
+        'myNumber': json['my_number'] == null ? undefined : json['my_number'],
+        'myString': json['my_string'] == null ? undefined : json['my_string'],
+        'myBoolean': json['my_boolean'] == null ? undefined : json['my_boolean'],
     };
 }
 
 export function OuterCompositeToJSON(value?: OuterComposite | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterObjectWithEnumProperty.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterObjectWithEnumProperty.ts
index 1182b747df28..3990064c3484 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterObjectWithEnumProperty.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/OuterObjectWithEnumProperty.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { OuterEnumInteger } from './OuterEnumInteger';
 import {
     OuterEnumIntegerFromJSON,
@@ -47,7 +47,7 @@ export function OuterObjectWithEnumPropertyFromJSON(json: any): OuterObjectWithE
 }
 
 export function OuterObjectWithEnumPropertyFromJSONTyped(json: any, ignoreDiscriminator: boolean): OuterObjectWithEnumProperty {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
@@ -57,11 +57,8 @@ export function OuterObjectWithEnumPropertyFromJSONTyped(json: any, ignoreDiscri
 }
 
 export function OuterObjectWithEnumPropertyToJSON(value?: OuterObjectWithEnumProperty | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ParentWithNullable.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ParentWithNullable.ts
index 829781419f2c..a593c5a384f9 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ParentWithNullable.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ParentWithNullable.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import {
      ChildWithNullableFromJSONTyped
 } from './index';
@@ -59,7 +59,7 @@ export function ParentWithNullableFromJSON(json: any): ParentWithNullable {
 }
 
 export function ParentWithNullableFromJSONTyped(json: any, ignoreDiscriminator: boolean): ParentWithNullable {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     if (!ignoreDiscriminator) {
@@ -69,17 +69,14 @@ export function ParentWithNullableFromJSONTyped(json: any, ignoreDiscriminator:
     }
     return {
         
-        'type': !exists(json, 'type') ? undefined : json['type'],
-        'nullableProperty': !exists(json, 'nullableProperty') ? undefined : json['nullableProperty'],
+        'type': json['type'] == null ? undefined : json['type'],
+        'nullableProperty': json['nullableProperty'] == null ? undefined : json['nullableProperty'],
     };
 }
 
 export function ParentWithNullableToJSON(value?: ParentWithNullable | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Pet.ts
index e53a44fc33f5..1f29621cb3b4 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Pet.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Category } from './Category';
 import {
     CategoryFromJSON,
@@ -96,26 +96,23 @@ export function PetFromJSON(json: any): Pet {
 }
 
 export function PetFromJSONTyped(json: any, ignoreDiscriminator: boolean): Pet {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'category': !exists(json, 'category') ? undefined : CategoryFromJSON(json['category']),
+        'id': json['id'] == null ? undefined : json['id'],
+        'category': json['category'] == null ? undefined : CategoryFromJSON(json['category']),
         'name': json['name'],
         'photoUrls': json['photoUrls'],
-        'tags': !exists(json, 'tags') ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
-        'status': !exists(json, 'status') ? undefined : json['status'],
+        'tags': json['tags'] == null ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
+        'status': json['status'] == null ? undefined : json['status'],
     };
 }
 
 export function PetToJSON(value?: Pet | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
@@ -123,7 +120,7 @@ export function PetToJSON(value?: Pet | null): any {
         'category': CategoryToJSON(value['category']),
         'name': value['name'],
         'photoUrls': Array.from(value['photoUrls'] as Set<any>),
-        'tags': !exists(value, 'tags') ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
+        'tags': value['tags'] == null ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
         'status': value['status'],
     };
 }
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ReadOnlyFirst.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ReadOnlyFirst.ts
index 97c192be7826..942bcbd883ac 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ReadOnlyFirst.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/ReadOnlyFirst.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -45,22 +45,19 @@ export function ReadOnlyFirstFromJSON(json: any): ReadOnlyFirst {
 }
 
 export function ReadOnlyFirstFromJSONTyped(json: any, ignoreDiscriminator: boolean): ReadOnlyFirst {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'bar': !exists(json, 'bar') ? undefined : json['bar'],
-        'baz': !exists(json, 'baz') ? undefined : json['baz'],
+        'bar': json['bar'] == null ? undefined : json['bar'],
+        'baz': json['baz'] == null ? undefined : json['baz'],
     };
 }
 
 export function ReadOnlyFirstToJSON(value?: ReadOnlyFirst | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Return.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Return.ts
index 9d2a3787c26d..68b15542cbbd 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Return.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Return.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Model for testing reserved words
  * @export
@@ -39,21 +39,18 @@ export function ReturnFromJSON(json: any): Return {
 }
 
 export function ReturnFromJSONTyped(json: any, ignoreDiscriminator: boolean): Return {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        '_return': !exists(json, 'return') ? undefined : json['return'],
+        '_return': json['return'] == null ? undefined : json['return'],
     };
 }
 
 export function ReturnToJSON(value?: Return | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/SpecialModelName.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/SpecialModelName.ts
index 414fcb168909..b302e6f03627 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/SpecialModelName.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/SpecialModelName.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function SpecialModelNameFromJSON(json: any): SpecialModelName {
 }
 
 export function SpecialModelNameFromJSONTyped(json: any, ignoreDiscriminator: boolean): SpecialModelName {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        '$specialPropertyName': !exists(json, '$special[property.name]') ? undefined : json['$special[property.name]'],
+        '$specialPropertyName': json['$special[property.name]'] == null ? undefined : json['$special[property.name]'],
     };
 }
 
 export function SpecialModelNameToJSON(value?: SpecialModelName | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Tag.ts
index 446a1d0da769..4ae4af399fd8 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Tag.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -45,22 +45,19 @@ export function TagFromJSON(json: any): Tag {
 }
 
 export function TagFromJSONTyped(json: any, ignoreDiscriminator: boolean): Tag {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function TagToJSON(value?: Tag | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/TestInlineFreeformAdditionalPropertiesRequest.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/TestInlineFreeformAdditionalPropertiesRequest.ts
index c3c3a41f093f..acc16c94f495 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/TestInlineFreeformAdditionalPropertiesRequest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/TestInlineFreeformAdditionalPropertiesRequest.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -40,22 +40,19 @@ export function TestInlineFreeformAdditionalPropertiesRequestFromJSON(json: any)
 }
 
 export function TestInlineFreeformAdditionalPropertiesRequestFromJSONTyped(json: any, ignoreDiscriminator: boolean): TestInlineFreeformAdditionalPropertiesRequest {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
             ...json,
-        'someProperty': !exists(json, 'someProperty') ? undefined : json['someProperty'],
+        'someProperty': json['someProperty'] == null ? undefined : json['someProperty'],
     };
 }
 
 export function TestInlineFreeformAdditionalPropertiesRequestToJSON(value?: TestInlineFreeformAdditionalPropertiesRequest | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/User.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/User.ts
index ed2c73a101e7..c85ea3d6ed63 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/User.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -81,28 +81,25 @@ export function UserFromJSON(json: any): User {
 }
 
 export function UserFromJSONTyped(json: any, ignoreDiscriminator: boolean): User {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'username': !exists(json, 'username') ? undefined : json['username'],
-        'firstName': !exists(json, 'firstName') ? undefined : json['firstName'],
-        'lastName': !exists(json, 'lastName') ? undefined : json['lastName'],
-        'email': !exists(json, 'email') ? undefined : json['email'],
-        'password': !exists(json, 'password') ? undefined : json['password'],
-        'phone': !exists(json, 'phone') ? undefined : json['phone'],
-        'userStatus': !exists(json, 'userStatus') ? undefined : json['userStatus'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'username': json['username'] == null ? undefined : json['username'],
+        'firstName': json['firstName'] == null ? undefined : json['firstName'],
+        'lastName': json['lastName'] == null ? undefined : json['lastName'],
+        'email': json['email'] == null ? undefined : json['email'],
+        'password': json['password'] == null ? undefined : json['password'],
+        'phone': json['phone'] == null ? undefined : json['phone'],
+        'userStatus': json['userStatus'] == null ? undefined : json['userStatus'],
     };
 }
 
 export function UserToJSON(value?: User | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts
index 11b3fb68ebf9..ed35f30ef7a5 100644
--- a/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default-v3.0/runtime.ts
@@ -310,11 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/default/apis/PetApi.ts b/samples/client/petstore/typescript-fetch/builds/default/apis/PetApi.ts
index 87cab8f993a2..22062ecf531f 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/apis/PetApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/apis/PetApi.ts
@@ -71,7 +71,7 @@ export class PetApi extends runtime.BaseAPI {
      * Add a new pet to the store
      */
     async addPetRaw(requestParameters: AddPetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling addPet().'
@@ -111,7 +111,7 @@ export class PetApi extends runtime.BaseAPI {
      * Deletes a pet
      */
     async deletePetRaw(requestParameters: DeletePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling deletePet().'
@@ -122,7 +122,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'apiKey')) {
+        if (requestParameters['apiKey'] != null) {
             headerParameters['api_key'] = String(requestParameters['apiKey']);
         }
 
@@ -153,7 +153,7 @@ export class PetApi extends runtime.BaseAPI {
      * Finds Pets by status
      */
     async findPetsByStatusRaw(requestParameters: FindPetsByStatusRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] == null) {
             throw new runtime.RequiredError(
                 'status',
                 'Required parameter "status" was null or undefined when calling findPetsByStatus().'
@@ -162,7 +162,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             queryParameters['status'] = requestParameters['status']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -198,7 +198,7 @@ export class PetApi extends runtime.BaseAPI {
      * @deprecated
      */
     async findPetsByTagsRaw(requestParameters: FindPetsByTagsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] == null) {
             throw new runtime.RequiredError(
                 'tags',
                 'Required parameter "tags" was null or undefined when calling findPetsByTags().'
@@ -207,7 +207,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] != null) {
             queryParameters['tags'] = requestParameters['tags']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -243,7 +243,7 @@ export class PetApi extends runtime.BaseAPI {
      * Find pet by ID
      */
     async getPetByIdRaw(requestParameters: GetPetByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Pet>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling getPetById().'
@@ -281,7 +281,7 @@ export class PetApi extends runtime.BaseAPI {
      * Update an existing pet
      */
     async updatePetRaw(requestParameters: UpdatePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updatePet().'
@@ -321,7 +321,7 @@ export class PetApi extends runtime.BaseAPI {
      * Updates a pet in the store with form data
      */
     async updatePetWithFormRaw(requestParameters: UpdatePetWithFormRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling updatePetWithForm().'
@@ -351,11 +351,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'name')) {
+        if (requestParameters['name'] != null) {
             formParams.append('name', requestParameters['name'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             formParams.append('status', requestParameters['status'] as any);
         }
 
@@ -381,7 +381,7 @@ export class PetApi extends runtime.BaseAPI {
      * uploads an image
      */
     async uploadFileRaw(requestParameters: UploadFileRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<ModelApiResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling uploadFile().'
@@ -413,11 +413,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'additionalMetadata')) {
+        if (requestParameters['additionalMetadata'] != null) {
             formParams.append('additionalMetadata', requestParameters['additionalMetadata'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'file')) {
+        if (requestParameters['file'] != null) {
             formParams.append('file', requestParameters['file'] as any);
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/default/apis/StoreApi.ts b/samples/client/petstore/typescript-fetch/builds/default/apis/StoreApi.ts
index 74fc267af7da..49163cc4b463 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/apis/StoreApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/apis/StoreApi.ts
@@ -44,7 +44,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Delete purchase order by ID
      */
     async deleteOrderRaw(requestParameters: DeleteOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling deleteOrder().'
@@ -110,7 +110,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Find purchase order by ID
      */
     async getOrderByIdRaw(requestParameters: GetOrderByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling getOrderById().'
@@ -144,7 +144,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Place an order for a pet
      */
     async placeOrderRaw(requestParameters: PlaceOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling placeOrder().'
diff --git a/samples/client/petstore/typescript-fetch/builds/default/apis/UserApi.ts b/samples/client/petstore/typescript-fetch/builds/default/apis/UserApi.ts
index 565ceefcaab9..03a5448df528 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/apis/UserApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/apis/UserApi.ts
@@ -62,7 +62,7 @@ export class UserApi extends runtime.BaseAPI {
      * Create user
      */
     async createUserRaw(requestParameters: CreateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUser().'
@@ -98,7 +98,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithArrayInputRaw(requestParameters: CreateUsersWithArrayInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithArrayInput().'
@@ -133,7 +133,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithListInputRaw(requestParameters: CreateUsersWithListInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithListInput().'
@@ -169,7 +169,7 @@ export class UserApi extends runtime.BaseAPI {
      * Delete user
      */
     async deleteUserRaw(requestParameters: DeleteUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling deleteUser().'
@@ -202,7 +202,7 @@ export class UserApi extends runtime.BaseAPI {
      * Get user by user name
      */
     async getUserByNameRaw(requestParameters: GetUserByNameRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<User>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling getUserByName().'
@@ -235,14 +235,14 @@ export class UserApi extends runtime.BaseAPI {
      * Logs user into the system
      */
     async loginUserRaw(requestParameters: LoginUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<string>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling loginUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] == null) {
             throw new runtime.RequiredError(
                 'password',
                 'Required parameter "password" was null or undefined when calling loginUser().'
@@ -251,11 +251,11 @@ export class UserApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] != null) {
             queryParameters['username'] = requestParameters['username'];
         }
 
-        if (runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] != null) {
             queryParameters['password'] = requestParameters['password'];
         }
 
@@ -313,14 +313,14 @@ export class UserApi extends runtime.BaseAPI {
      * Updated user
      */
     async updateUserRaw(requestParameters: UpdateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling updateUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updateUser().'
diff --git a/samples/client/petstore/typescript-fetch/builds/default/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/default/models/Category.ts
index 001bc135bb96..19e40eaf4b86 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/models/Category.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A category for a pet
  * @export
@@ -45,22 +45,19 @@ export function CategoryFromJSON(json: any): Category {
 }
 
 export function CategoryFromJSONTyped(json: any, ignoreDiscriminator: boolean): Category {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function CategoryToJSON(value?: Category | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/default/models/ModelApiResponse.ts
index a7fc63d81f1b..4496efe73f05 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/models/ModelApiResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Describes the result of uploading an image resource
  * @export
@@ -51,23 +51,20 @@ export function ModelApiResponseFromJSON(json: any): ModelApiResponse {
 }
 
 export function ModelApiResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): ModelApiResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'code': !exists(json, 'code') ? undefined : json['code'],
-        'type': !exists(json, 'type') ? undefined : json['type'],
-        'message': !exists(json, 'message') ? undefined : json['message'],
+        'code': json['code'] == null ? undefined : json['code'],
+        'type': json['type'] == null ? undefined : json['type'],
+        'message': json['message'] == null ? undefined : json['message'],
     };
 }
 
 export function ModelApiResponseToJSON(value?: ModelApiResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/default/models/Order.ts
index 9e12fc242793..948cda435c8a 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/models/Order.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * An order for a pets from the pet store
  * @export
@@ -81,33 +81,30 @@ export function OrderFromJSON(json: any): Order {
 }
 
 export function OrderFromJSONTyped(json: any, ignoreDiscriminator: boolean): Order {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'petId': !exists(json, 'petId') ? undefined : json['petId'],
-        'quantity': !exists(json, 'quantity') ? undefined : json['quantity'],
-        'shipDate': !exists(json, 'shipDate') ? undefined : (new Date(json['shipDate'])),
-        'status': !exists(json, 'status') ? undefined : json['status'],
-        'complete': !exists(json, 'complete') ? undefined : json['complete'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'petId': json['petId'] == null ? undefined : json['petId'],
+        'quantity': json['quantity'] == null ? undefined : json['quantity'],
+        'shipDate': json['shipDate'] == null ? undefined : (new Date(json['shipDate'])),
+        'status': json['status'] == null ? undefined : json['status'],
+        'complete': json['complete'] == null ? undefined : json['complete'],
     };
 }
 
 export function OrderToJSON(value?: Order | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': !exists(value, 'shipDate') ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
         'status': value['status'],
         'complete': value['complete'],
     };
diff --git a/samples/client/petstore/typescript-fetch/builds/default/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/default/models/Pet.ts
index 8906602f6f4b..3c60d980598b 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/models/Pet.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Category } from './Category';
 import {
     CategoryFromJSON,
@@ -96,26 +96,23 @@ export function PetFromJSON(json: any): Pet {
 }
 
 export function PetFromJSONTyped(json: any, ignoreDiscriminator: boolean): Pet {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'category': !exists(json, 'category') ? undefined : CategoryFromJSON(json['category']),
+        'id': json['id'] == null ? undefined : json['id'],
+        'category': json['category'] == null ? undefined : CategoryFromJSON(json['category']),
         'name': json['name'],
         'photoUrls': json['photoUrls'],
-        'tags': !exists(json, 'tags') ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
-        'status': !exists(json, 'status') ? undefined : json['status'],
+        'tags': json['tags'] == null ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
+        'status': json['status'] == null ? undefined : json['status'],
     };
 }
 
 export function PetToJSON(value?: Pet | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
@@ -123,7 +120,7 @@ export function PetToJSON(value?: Pet | null): any {
         'category': CategoryToJSON(value['category']),
         'name': value['name'],
         'photoUrls': value['photoUrls'],
-        'tags': !exists(value, 'tags') ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
+        'tags': value['tags'] == null ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
         'status': value['status'],
     };
 }
diff --git a/samples/client/petstore/typescript-fetch/builds/default/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/default/models/Tag.ts
index 24a25f383dba..0a5bdbdde07c 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/models/Tag.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A tag for a pet
  * @export
@@ -45,22 +45,19 @@ export function TagFromJSON(json: any): Tag {
 }
 
 export function TagFromJSONTyped(json: any, ignoreDiscriminator: boolean): Tag {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function TagToJSON(value?: Tag | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default/models/User.ts b/samples/client/petstore/typescript-fetch/builds/default/models/User.ts
index 00ab35b80563..3924497622f9 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/models/User.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A User who is purchasing from the pet store
  * @export
@@ -81,28 +81,25 @@ export function UserFromJSON(json: any): User {
 }
 
 export function UserFromJSONTyped(json: any, ignoreDiscriminator: boolean): User {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'username': !exists(json, 'username') ? undefined : json['username'],
-        'firstName': !exists(json, 'firstName') ? undefined : json['firstName'],
-        'lastName': !exists(json, 'lastName') ? undefined : json['lastName'],
-        'email': !exists(json, 'email') ? undefined : json['email'],
-        'password': !exists(json, 'password') ? undefined : json['password'],
-        'phone': !exists(json, 'phone') ? undefined : json['phone'],
-        'userStatus': !exists(json, 'userStatus') ? undefined : json['userStatus'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'username': json['username'] == null ? undefined : json['username'],
+        'firstName': json['firstName'] == null ? undefined : json['firstName'],
+        'lastName': json['lastName'] == null ? undefined : json['lastName'],
+        'email': json['email'] == null ? undefined : json['email'],
+        'password': json['password'] == null ? undefined : json['password'],
+        'phone': json['phone'] == null ? undefined : json['phone'],
+        'userStatus': json['userStatus'] == null ? undefined : json['userStatus'],
     };
 }
 
 export function UserToJSON(value?: User | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
index 813bf0fb4dfe..2c4103a8908e 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/runtime.ts
@@ -310,11 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/enum/apis/DefaultApi.ts b/samples/client/petstore/typescript-fetch/builds/enum/apis/DefaultApi.ts
index f54273179d37..2ce4f1d2cf45 100644
--- a/samples/client/petstore/typescript-fetch/builds/enum/apis/DefaultApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/enum/apis/DefaultApi.ts
@@ -63,19 +63,19 @@ export class DefaultApi extends runtime.BaseAPI {
     async fakeEnumRequestGetInlineRaw(requestParameters: FakeEnumRequestGetInlineRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<FakeEnumRequestGetInline200Response>> {
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'stringEnum')) {
+        if (requestParameters['stringEnum'] != null) {
             queryParameters['string-enum'] = requestParameters['stringEnum'];
         }
 
-        if (runtime.exists(requestParameters, 'nullableStringEnum')) {
+        if (requestParameters['nullableStringEnum'] != null) {
             queryParameters['nullable-string-enum'] = requestParameters['nullableStringEnum'];
         }
 
-        if (runtime.exists(requestParameters, 'numberEnum')) {
+        if (requestParameters['numberEnum'] != null) {
             queryParameters['number-enum'] = requestParameters['numberEnum'];
         }
 
-        if (runtime.exists(requestParameters, 'nullableNumberEnum')) {
+        if (requestParameters['nullableNumberEnum'] != null) {
             queryParameters['nullable-number-enum'] = requestParameters['nullableNumberEnum'];
         }
 
@@ -103,19 +103,19 @@ export class DefaultApi extends runtime.BaseAPI {
     async fakeEnumRequestGetRefRaw(requestParameters: FakeEnumRequestGetRefRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<EnumPatternObject>> {
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'stringEnum')) {
+        if (requestParameters['stringEnum'] != null) {
             queryParameters['string-enum'] = requestParameters['stringEnum'];
         }
 
-        if (runtime.exists(requestParameters, 'nullableStringEnum')) {
+        if (requestParameters['nullableStringEnum'] != null) {
             queryParameters['nullable-string-enum'] = requestParameters['nullableStringEnum'];
         }
 
-        if (runtime.exists(requestParameters, 'numberEnum')) {
+        if (requestParameters['numberEnum'] != null) {
             queryParameters['number-enum'] = requestParameters['numberEnum'];
         }
 
-        if (runtime.exists(requestParameters, 'nullableNumberEnum')) {
+        if (requestParameters['nullableNumberEnum'] != null) {
             queryParameters['nullable-number-enum'] = requestParameters['nullableNumberEnum'];
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/enum/models/EnumPatternObject.ts b/samples/client/petstore/typescript-fetch/builds/enum/models/EnumPatternObject.ts
index 38d190ba0f39..a902fb9c9af3 100644
--- a/samples/client/petstore/typescript-fetch/builds/enum/models/EnumPatternObject.ts
+++ b/samples/client/petstore/typescript-fetch/builds/enum/models/EnumPatternObject.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { NumberEnum } from './NumberEnum';
 import {
     NumberEnumFromJSON,
@@ -70,24 +70,21 @@ export function EnumPatternObjectFromJSON(json: any): EnumPatternObject {
 }
 
 export function EnumPatternObjectFromJSONTyped(json: any, ignoreDiscriminator: boolean): EnumPatternObject {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'stringEnum': !exists(json, 'string-enum') ? undefined : StringEnumFromJSON(json['string-enum']),
-        'nullableStringEnum': !exists(json, 'nullable-string-enum') ? undefined : StringEnumFromJSON(json['nullable-string-enum']),
-        'numberEnum': !exists(json, 'number-enum') ? undefined : NumberEnumFromJSON(json['number-enum']),
-        'nullableNumberEnum': !exists(json, 'nullable-number-enum') ? undefined : NumberEnumFromJSON(json['nullable-number-enum']),
+        'stringEnum': json['string-enum'] == null ? undefined : StringEnumFromJSON(json['string-enum']),
+        'nullableStringEnum': json['nullable-string-enum'] == null ? undefined : StringEnumFromJSON(json['nullable-string-enum']),
+        'numberEnum': json['number-enum'] == null ? undefined : NumberEnumFromJSON(json['number-enum']),
+        'nullableNumberEnum': json['nullable-number-enum'] == null ? undefined : NumberEnumFromJSON(json['nullable-number-enum']),
     };
 }
 
 export function EnumPatternObjectToJSON(value?: EnumPatternObject | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/enum/models/FakeEnumRequestGetInline200Response.ts b/samples/client/petstore/typescript-fetch/builds/enum/models/FakeEnumRequestGetInline200Response.ts
index 3ce94b18453f..0e53bcfd09d6 100644
--- a/samples/client/petstore/typescript-fetch/builds/enum/models/FakeEnumRequestGetInline200Response.ts
+++ b/samples/client/petstore/typescript-fetch/builds/enum/models/FakeEnumRequestGetInline200Response.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -99,24 +99,21 @@ export function FakeEnumRequestGetInline200ResponseFromJSON(json: any): FakeEnum
 }
 
 export function FakeEnumRequestGetInline200ResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): FakeEnumRequestGetInline200Response {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'stringEnum': !exists(json, 'string-enum') ? undefined : json['string-enum'],
-        'nullableStringEnum': !exists(json, 'nullable-string-enum') ? undefined : json['nullable-string-enum'],
-        'numberEnum': !exists(json, 'number-enum') ? undefined : json['number-enum'],
-        'nullableNumberEnum': !exists(json, 'nullable-number-enum') ? undefined : json['nullable-number-enum'],
+        'stringEnum': json['string-enum'] == null ? undefined : json['string-enum'],
+        'nullableStringEnum': json['nullable-string-enum'] == null ? undefined : json['nullable-string-enum'],
+        'numberEnum': json['number-enum'] == null ? undefined : json['number-enum'],
+        'nullableNumberEnum': json['nullable-number-enum'] == null ? undefined : json['nullable-number-enum'],
     };
 }
 
 export function FakeEnumRequestGetInline200ResponseToJSON(value?: FakeEnumRequestGetInline200Response | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts b/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
index 4f1da922f6df..7f99552816fe 100644
--- a/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/enum/runtime.ts
@@ -310,11 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/apis/PetApi.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/apis/PetApi.ts
index 87cab8f993a2..22062ecf531f 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/apis/PetApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/apis/PetApi.ts
@@ -71,7 +71,7 @@ export class PetApi extends runtime.BaseAPI {
      * Add a new pet to the store
      */
     async addPetRaw(requestParameters: AddPetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling addPet().'
@@ -111,7 +111,7 @@ export class PetApi extends runtime.BaseAPI {
      * Deletes a pet
      */
     async deletePetRaw(requestParameters: DeletePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling deletePet().'
@@ -122,7 +122,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'apiKey')) {
+        if (requestParameters['apiKey'] != null) {
             headerParameters['api_key'] = String(requestParameters['apiKey']);
         }
 
@@ -153,7 +153,7 @@ export class PetApi extends runtime.BaseAPI {
      * Finds Pets by status
      */
     async findPetsByStatusRaw(requestParameters: FindPetsByStatusRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] == null) {
             throw new runtime.RequiredError(
                 'status',
                 'Required parameter "status" was null or undefined when calling findPetsByStatus().'
@@ -162,7 +162,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             queryParameters['status'] = requestParameters['status']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -198,7 +198,7 @@ export class PetApi extends runtime.BaseAPI {
      * @deprecated
      */
     async findPetsByTagsRaw(requestParameters: FindPetsByTagsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] == null) {
             throw new runtime.RequiredError(
                 'tags',
                 'Required parameter "tags" was null or undefined when calling findPetsByTags().'
@@ -207,7 +207,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] != null) {
             queryParameters['tags'] = requestParameters['tags']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -243,7 +243,7 @@ export class PetApi extends runtime.BaseAPI {
      * Find pet by ID
      */
     async getPetByIdRaw(requestParameters: GetPetByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Pet>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling getPetById().'
@@ -281,7 +281,7 @@ export class PetApi extends runtime.BaseAPI {
      * Update an existing pet
      */
     async updatePetRaw(requestParameters: UpdatePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updatePet().'
@@ -321,7 +321,7 @@ export class PetApi extends runtime.BaseAPI {
      * Updates a pet in the store with form data
      */
     async updatePetWithFormRaw(requestParameters: UpdatePetWithFormRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling updatePetWithForm().'
@@ -351,11 +351,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'name')) {
+        if (requestParameters['name'] != null) {
             formParams.append('name', requestParameters['name'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             formParams.append('status', requestParameters['status'] as any);
         }
 
@@ -381,7 +381,7 @@ export class PetApi extends runtime.BaseAPI {
      * uploads an image
      */
     async uploadFileRaw(requestParameters: UploadFileRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<ModelApiResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling uploadFile().'
@@ -413,11 +413,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'additionalMetadata')) {
+        if (requestParameters['additionalMetadata'] != null) {
             formParams.append('additionalMetadata', requestParameters['additionalMetadata'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'file')) {
+        if (requestParameters['file'] != null) {
             formParams.append('file', requestParameters['file'] as any);
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/apis/StoreApi.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/apis/StoreApi.ts
index 74fc267af7da..49163cc4b463 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/apis/StoreApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/apis/StoreApi.ts
@@ -44,7 +44,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Delete purchase order by ID
      */
     async deleteOrderRaw(requestParameters: DeleteOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling deleteOrder().'
@@ -110,7 +110,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Find purchase order by ID
      */
     async getOrderByIdRaw(requestParameters: GetOrderByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling getOrderById().'
@@ -144,7 +144,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Place an order for a pet
      */
     async placeOrderRaw(requestParameters: PlaceOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling placeOrder().'
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/apis/UserApi.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/apis/UserApi.ts
index 565ceefcaab9..03a5448df528 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/apis/UserApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/apis/UserApi.ts
@@ -62,7 +62,7 @@ export class UserApi extends runtime.BaseAPI {
      * Create user
      */
     async createUserRaw(requestParameters: CreateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUser().'
@@ -98,7 +98,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithArrayInputRaw(requestParameters: CreateUsersWithArrayInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithArrayInput().'
@@ -133,7 +133,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithListInputRaw(requestParameters: CreateUsersWithListInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithListInput().'
@@ -169,7 +169,7 @@ export class UserApi extends runtime.BaseAPI {
      * Delete user
      */
     async deleteUserRaw(requestParameters: DeleteUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling deleteUser().'
@@ -202,7 +202,7 @@ export class UserApi extends runtime.BaseAPI {
      * Get user by user name
      */
     async getUserByNameRaw(requestParameters: GetUserByNameRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<User>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling getUserByName().'
@@ -235,14 +235,14 @@ export class UserApi extends runtime.BaseAPI {
      * Logs user into the system
      */
     async loginUserRaw(requestParameters: LoginUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<string>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling loginUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] == null) {
             throw new runtime.RequiredError(
                 'password',
                 'Required parameter "password" was null or undefined when calling loginUser().'
@@ -251,11 +251,11 @@ export class UserApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] != null) {
             queryParameters['username'] = requestParameters['username'];
         }
 
-        if (runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] != null) {
             queryParameters['password'] = requestParameters['password'];
         }
 
@@ -313,14 +313,14 @@ export class UserApi extends runtime.BaseAPI {
      * Updated user
      */
     async updateUserRaw(requestParameters: UpdateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling updateUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updateUser().'
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Category.ts
index 001bc135bb96..19e40eaf4b86 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Category.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A category for a pet
  * @export
@@ -45,22 +45,19 @@ export function CategoryFromJSON(json: any): Category {
 }
 
 export function CategoryFromJSONTyped(json: any, ignoreDiscriminator: boolean): Category {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function CategoryToJSON(value?: Category | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/ModelApiResponse.ts
index a7fc63d81f1b..4496efe73f05 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/ModelApiResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Describes the result of uploading an image resource
  * @export
@@ -51,23 +51,20 @@ export function ModelApiResponseFromJSON(json: any): ModelApiResponse {
 }
 
 export function ModelApiResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): ModelApiResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'code': !exists(json, 'code') ? undefined : json['code'],
-        'type': !exists(json, 'type') ? undefined : json['type'],
-        'message': !exists(json, 'message') ? undefined : json['message'],
+        'code': json['code'] == null ? undefined : json['code'],
+        'type': json['type'] == null ? undefined : json['type'],
+        'message': json['message'] == null ? undefined : json['message'],
     };
 }
 
 export function ModelApiResponseToJSON(value?: ModelApiResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Order.ts
index 9e12fc242793..948cda435c8a 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Order.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * An order for a pets from the pet store
  * @export
@@ -81,33 +81,30 @@ export function OrderFromJSON(json: any): Order {
 }
 
 export function OrderFromJSONTyped(json: any, ignoreDiscriminator: boolean): Order {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'petId': !exists(json, 'petId') ? undefined : json['petId'],
-        'quantity': !exists(json, 'quantity') ? undefined : json['quantity'],
-        'shipDate': !exists(json, 'shipDate') ? undefined : (new Date(json['shipDate'])),
-        'status': !exists(json, 'status') ? undefined : json['status'],
-        'complete': !exists(json, 'complete') ? undefined : json['complete'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'petId': json['petId'] == null ? undefined : json['petId'],
+        'quantity': json['quantity'] == null ? undefined : json['quantity'],
+        'shipDate': json['shipDate'] == null ? undefined : (new Date(json['shipDate'])),
+        'status': json['status'] == null ? undefined : json['status'],
+        'complete': json['complete'] == null ? undefined : json['complete'],
     };
 }
 
 export function OrderToJSON(value?: Order | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': !exists(value, 'shipDate') ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
         'status': value['status'],
         'complete': value['complete'],
     };
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Pet.ts
index 8906602f6f4b..3c60d980598b 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Pet.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Category } from './Category';
 import {
     CategoryFromJSON,
@@ -96,26 +96,23 @@ export function PetFromJSON(json: any): Pet {
 }
 
 export function PetFromJSONTyped(json: any, ignoreDiscriminator: boolean): Pet {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'category': !exists(json, 'category') ? undefined : CategoryFromJSON(json['category']),
+        'id': json['id'] == null ? undefined : json['id'],
+        'category': json['category'] == null ? undefined : CategoryFromJSON(json['category']),
         'name': json['name'],
         'photoUrls': json['photoUrls'],
-        'tags': !exists(json, 'tags') ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
-        'status': !exists(json, 'status') ? undefined : json['status'],
+        'tags': json['tags'] == null ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
+        'status': json['status'] == null ? undefined : json['status'],
     };
 }
 
 export function PetToJSON(value?: Pet | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
@@ -123,7 +120,7 @@ export function PetToJSON(value?: Pet | null): any {
         'category': CategoryToJSON(value['category']),
         'name': value['name'],
         'photoUrls': value['photoUrls'],
-        'tags': !exists(value, 'tags') ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
+        'tags': value['tags'] == null ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
         'status': value['status'],
     };
 }
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Tag.ts
index 24a25f383dba..0a5bdbdde07c 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/Tag.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A tag for a pet
  * @export
@@ -45,22 +45,19 @@ export function TagFromJSON(json: any): Tag {
 }
 
 export function TagFromJSONTyped(json: any, ignoreDiscriminator: boolean): Tag {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function TagToJSON(value?: Tag | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/User.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/User.ts
index 00ab35b80563..3924497622f9 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/models/User.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A User who is purchasing from the pet store
  * @export
@@ -81,28 +81,25 @@ export function UserFromJSON(json: any): User {
 }
 
 export function UserFromJSONTyped(json: any, ignoreDiscriminator: boolean): User {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'username': !exists(json, 'username') ? undefined : json['username'],
-        'firstName': !exists(json, 'firstName') ? undefined : json['firstName'],
-        'lastName': !exists(json, 'lastName') ? undefined : json['lastName'],
-        'email': !exists(json, 'email') ? undefined : json['email'],
-        'password': !exists(json, 'password') ? undefined : json['password'],
-        'phone': !exists(json, 'phone') ? undefined : json['phone'],
-        'userStatus': !exists(json, 'userStatus') ? undefined : json['userStatus'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'username': json['username'] == null ? undefined : json['username'],
+        'firstName': json['firstName'] == null ? undefined : json['firstName'],
+        'lastName': json['lastName'] == null ? undefined : json['lastName'],
+        'email': json['email'] == null ? undefined : json['email'],
+        'password': json['password'] == null ? undefined : json['password'],
+        'phone': json['phone'] == null ? undefined : json['phone'],
+        'userStatus': json['userStatus'] == null ? undefined : json['userStatus'],
     };
 }
 
 export function UserToJSON(value?: User | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts
index 813bf0fb4dfe..2c4103a8908e 100644
--- a/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/es6-target/src/runtime.ts
@@ -310,11 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/apis/PetApi.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/apis/PetApi.ts
index bfef64a126ac..66a4232c3893 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/apis/PetApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/apis/PetApi.ts
@@ -71,7 +71,7 @@ export class PetApi extends runtime.BaseAPI {
      * Add a new pet to the store
      */
     async addPetRaw(requestParameters: AddPetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling addPet().'
@@ -111,7 +111,7 @@ export class PetApi extends runtime.BaseAPI {
      * Deletes a pet
      */
     async deletePetRaw(requestParameters: DeletePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling deletePet().'
@@ -122,7 +122,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'apiKey')) {
+        if (requestParameters['apiKey'] != null) {
             headerParameters['api_key'] = String(requestParameters['apiKey']);
         }
 
@@ -153,7 +153,7 @@ export class PetApi extends runtime.BaseAPI {
      * Finds Pets by status
      */
     async findPetsByStatusRaw(requestParameters: FindPetsByStatusRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] == null) {
             throw new runtime.RequiredError(
                 'status',
                 'Required parameter "status" was null or undefined when calling findPetsByStatus().'
@@ -162,7 +162,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             queryParameters['status'] = requestParameters['status']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -198,7 +198,7 @@ export class PetApi extends runtime.BaseAPI {
      * @deprecated
      */
     async findPetsByTagsRaw(requestParameters: FindPetsByTagsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] == null) {
             throw new runtime.RequiredError(
                 'tags',
                 'Required parameter "tags" was null or undefined when calling findPetsByTags().'
@@ -207,7 +207,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] != null) {
             queryParameters['tags'] = requestParameters['tags']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -243,7 +243,7 @@ export class PetApi extends runtime.BaseAPI {
      * Find pet by ID
      */
     async getPetByIdRaw(requestParameters: GetPetByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Pet>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling getPetById().'
@@ -281,7 +281,7 @@ export class PetApi extends runtime.BaseAPI {
      * Update an existing pet
      */
     async updatePetRaw(requestParameters: UpdatePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updatePet().'
@@ -321,7 +321,7 @@ export class PetApi extends runtime.BaseAPI {
      * Updates a pet in the store with form data
      */
     async updatePetWithFormRaw(requestParameters: UpdatePetWithFormRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling updatePetWithForm().'
@@ -351,11 +351,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'name')) {
+        if (requestParameters['name'] != null) {
             formParams.append('name', requestParameters['name'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             formParams.append('status', requestParameters['status'] as any);
         }
 
@@ -381,7 +381,7 @@ export class PetApi extends runtime.BaseAPI {
      * uploads an image
      */
     async uploadFileRaw(requestParameters: UploadFileRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<ModelApiResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling uploadFile().'
@@ -413,11 +413,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'additionalMetadata')) {
+        if (requestParameters['additionalMetadata'] != null) {
             formParams.append('additionalMetadata', requestParameters['additionalMetadata'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'file')) {
+        if (requestParameters['file'] != null) {
             formParams.append('file', requestParameters['file'] as any);
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/apis/StoreApi.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/apis/StoreApi.ts
index 71f08a8daed8..8cf46da17dd8 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/apis/StoreApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/apis/StoreApi.ts
@@ -44,7 +44,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Delete purchase order by ID
      */
     async deleteOrderRaw(requestParameters: DeleteOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling deleteOrder().'
@@ -110,7 +110,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Find purchase order by ID
      */
     async getOrderByIdRaw(requestParameters: GetOrderByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling getOrderById().'
@@ -144,7 +144,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Place an order for a pet
      */
     async placeOrderRaw(requestParameters: PlaceOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling placeOrder().'
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/apis/UserApi.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/apis/UserApi.ts
index 49ed31f55b8f..b60bf74dc414 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/apis/UserApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/apis/UserApi.ts
@@ -62,7 +62,7 @@ export class UserApi extends runtime.BaseAPI {
      * Create user
      */
     async createUserRaw(requestParameters: CreateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUser().'
@@ -98,7 +98,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithArrayInputRaw(requestParameters: CreateUsersWithArrayInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithArrayInput().'
@@ -133,7 +133,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithListInputRaw(requestParameters: CreateUsersWithListInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithListInput().'
@@ -169,7 +169,7 @@ export class UserApi extends runtime.BaseAPI {
      * Delete user
      */
     async deleteUserRaw(requestParameters: DeleteUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling deleteUser().'
@@ -202,7 +202,7 @@ export class UserApi extends runtime.BaseAPI {
      * Get user by user name
      */
     async getUserByNameRaw(requestParameters: GetUserByNameRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<User>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling getUserByName().'
@@ -235,14 +235,14 @@ export class UserApi extends runtime.BaseAPI {
      * Logs user into the system
      */
     async loginUserRaw(requestParameters: LoginUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<string>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling loginUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] == null) {
             throw new runtime.RequiredError(
                 'password',
                 'Required parameter "password" was null or undefined when calling loginUser().'
@@ -251,11 +251,11 @@ export class UserApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] != null) {
             queryParameters['username'] = requestParameters['username'];
         }
 
-        if (runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] != null) {
             queryParameters['password'] = requestParameters['password'];
         }
 
@@ -313,14 +313,14 @@ export class UserApi extends runtime.BaseAPI {
      * Updated user
      */
     async updateUserRaw(requestParameters: UpdateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling updateUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updateUser().'
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Category.ts
index 001bc135bb96..19e40eaf4b86 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Category.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A category for a pet
  * @export
@@ -45,22 +45,19 @@ export function CategoryFromJSON(json: any): Category {
 }
 
 export function CategoryFromJSONTyped(json: any, ignoreDiscriminator: boolean): Category {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function CategoryToJSON(value?: Category | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/ModelApiResponse.ts
index a7fc63d81f1b..4496efe73f05 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/ModelApiResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Describes the result of uploading an image resource
  * @export
@@ -51,23 +51,20 @@ export function ModelApiResponseFromJSON(json: any): ModelApiResponse {
 }
 
 export function ModelApiResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): ModelApiResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'code': !exists(json, 'code') ? undefined : json['code'],
-        'type': !exists(json, 'type') ? undefined : json['type'],
-        'message': !exists(json, 'message') ? undefined : json['message'],
+        'code': json['code'] == null ? undefined : json['code'],
+        'type': json['type'] == null ? undefined : json['type'],
+        'message': json['message'] == null ? undefined : json['message'],
     };
 }
 
 export function ModelApiResponseToJSON(value?: ModelApiResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Order.ts
index 9e12fc242793..948cda435c8a 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Order.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * An order for a pets from the pet store
  * @export
@@ -81,33 +81,30 @@ export function OrderFromJSON(json: any): Order {
 }
 
 export function OrderFromJSONTyped(json: any, ignoreDiscriminator: boolean): Order {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'petId': !exists(json, 'petId') ? undefined : json['petId'],
-        'quantity': !exists(json, 'quantity') ? undefined : json['quantity'],
-        'shipDate': !exists(json, 'shipDate') ? undefined : (new Date(json['shipDate'])),
-        'status': !exists(json, 'status') ? undefined : json['status'],
-        'complete': !exists(json, 'complete') ? undefined : json['complete'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'petId': json['petId'] == null ? undefined : json['petId'],
+        'quantity': json['quantity'] == null ? undefined : json['quantity'],
+        'shipDate': json['shipDate'] == null ? undefined : (new Date(json['shipDate'])),
+        'status': json['status'] == null ? undefined : json['status'],
+        'complete': json['complete'] == null ? undefined : json['complete'],
     };
 }
 
 export function OrderToJSON(value?: Order | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': !exists(value, 'shipDate') ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
         'status': value['status'],
         'complete': value['complete'],
     };
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Pet.ts
index 8906602f6f4b..3c60d980598b 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Pet.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Category } from './Category';
 import {
     CategoryFromJSON,
@@ -96,26 +96,23 @@ export function PetFromJSON(json: any): Pet {
 }
 
 export function PetFromJSONTyped(json: any, ignoreDiscriminator: boolean): Pet {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'category': !exists(json, 'category') ? undefined : CategoryFromJSON(json['category']),
+        'id': json['id'] == null ? undefined : json['id'],
+        'category': json['category'] == null ? undefined : CategoryFromJSON(json['category']),
         'name': json['name'],
         'photoUrls': json['photoUrls'],
-        'tags': !exists(json, 'tags') ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
-        'status': !exists(json, 'status') ? undefined : json['status'],
+        'tags': json['tags'] == null ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
+        'status': json['status'] == null ? undefined : json['status'],
     };
 }
 
 export function PetToJSON(value?: Pet | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
@@ -123,7 +120,7 @@ export function PetToJSON(value?: Pet | null): any {
         'category': CategoryToJSON(value['category']),
         'name': value['name'],
         'photoUrls': value['photoUrls'],
-        'tags': !exists(value, 'tags') ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
+        'tags': value['tags'] == null ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
         'status': value['status'],
     };
 }
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Tag.ts
index 24a25f383dba..0a5bdbdde07c 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/Tag.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A tag for a pet
  * @export
@@ -45,22 +45,19 @@ export function TagFromJSON(json: any): Tag {
 }
 
 export function TagFromJSONTyped(json: any, ignoreDiscriminator: boolean): Tag {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function TagToJSON(value?: Tag | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/User.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/User.ts
index 00ab35b80563..3924497622f9 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/models/User.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A User who is purchasing from the pet store
  * @export
@@ -81,28 +81,25 @@ export function UserFromJSON(json: any): User {
 }
 
 export function UserFromJSONTyped(json: any, ignoreDiscriminator: boolean): User {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'username': !exists(json, 'username') ? undefined : json['username'],
-        'firstName': !exists(json, 'firstName') ? undefined : json['firstName'],
-        'lastName': !exists(json, 'lastName') ? undefined : json['lastName'],
-        'email': !exists(json, 'email') ? undefined : json['email'],
-        'password': !exists(json, 'password') ? undefined : json['password'],
-        'phone': !exists(json, 'phone') ? undefined : json['phone'],
-        'userStatus': !exists(json, 'userStatus') ? undefined : json['userStatus'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'username': json['username'] == null ? undefined : json['username'],
+        'firstName': json['firstName'] == null ? undefined : json['firstName'],
+        'lastName': json['lastName'] == null ? undefined : json['lastName'],
+        'email': json['email'] == null ? undefined : json['email'],
+        'password': json['password'] == null ? undefined : json['password'],
+        'phone': json['phone'] == null ? undefined : json['phone'],
+        'userStatus': json['userStatus'] == null ? undefined : json['userStatus'],
     };
 }
 
 export function UserToJSON(value?: User | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
index 813bf0fb4dfe..2c4103a8908e 100644
--- a/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/multiple-parameters/runtime.ts
@@ -310,11 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/apis/PetApi.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/apis/PetApi.ts
index c1e5d3f61069..4776d47251c8 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/apis/PetApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/apis/PetApi.ts
@@ -71,7 +71,7 @@ export class PetApi extends runtime.BaseAPI {
      * Add a new pet to the store
      */
     async addPetRaw(requestParameters: PetApiAddPetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling addPet().'
@@ -111,7 +111,7 @@ export class PetApi extends runtime.BaseAPI {
      * Deletes a pet
      */
     async deletePetRaw(requestParameters: PetApiDeletePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling deletePet().'
@@ -122,7 +122,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'apiKey')) {
+        if (requestParameters['apiKey'] != null) {
             headerParameters['api_key'] = String(requestParameters['apiKey']);
         }
 
@@ -153,7 +153,7 @@ export class PetApi extends runtime.BaseAPI {
      * Finds Pets by status
      */
     async findPetsByStatusRaw(requestParameters: PetApiFindPetsByStatusRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] == null) {
             throw new runtime.RequiredError(
                 'status',
                 'Required parameter "status" was null or undefined when calling findPetsByStatus().'
@@ -162,7 +162,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             queryParameters['status'] = requestParameters['status']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -198,7 +198,7 @@ export class PetApi extends runtime.BaseAPI {
      * @deprecated
      */
     async findPetsByTagsRaw(requestParameters: PetApiFindPetsByTagsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] == null) {
             throw new runtime.RequiredError(
                 'tags',
                 'Required parameter "tags" was null or undefined when calling findPetsByTags().'
@@ -207,7 +207,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] != null) {
             queryParameters['tags'] = requestParameters['tags']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -243,7 +243,7 @@ export class PetApi extends runtime.BaseAPI {
      * Find pet by ID
      */
     async getPetByIdRaw(requestParameters: PetApiGetPetByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Pet>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling getPetById().'
@@ -281,7 +281,7 @@ export class PetApi extends runtime.BaseAPI {
      * Update an existing pet
      */
     async updatePetRaw(requestParameters: PetApiUpdatePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updatePet().'
@@ -321,7 +321,7 @@ export class PetApi extends runtime.BaseAPI {
      * Updates a pet in the store with form data
      */
     async updatePetWithFormRaw(requestParameters: PetApiUpdatePetWithFormRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling updatePetWithForm().'
@@ -351,11 +351,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'name')) {
+        if (requestParameters['name'] != null) {
             formParams.append('name', requestParameters['name'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             formParams.append('status', requestParameters['status'] as any);
         }
 
@@ -381,7 +381,7 @@ export class PetApi extends runtime.BaseAPI {
      * uploads an image
      */
     async uploadFileRaw(requestParameters: PetApiUploadFileRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<ModelApiResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling uploadFile().'
@@ -413,11 +413,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'additionalMetadata')) {
+        if (requestParameters['additionalMetadata'] != null) {
             formParams.append('additionalMetadata', requestParameters['additionalMetadata'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'file')) {
+        if (requestParameters['file'] != null) {
             formParams.append('file', requestParameters['file'] as any);
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/apis/StoreApi.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/apis/StoreApi.ts
index c4166be39728..58434bd8740c 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/apis/StoreApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/apis/StoreApi.ts
@@ -44,7 +44,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Delete purchase order by ID
      */
     async deleteOrderRaw(requestParameters: StoreApiDeleteOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling deleteOrder().'
@@ -110,7 +110,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Find purchase order by ID
      */
     async getOrderByIdRaw(requestParameters: StoreApiGetOrderByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling getOrderById().'
@@ -144,7 +144,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Place an order for a pet
      */
     async placeOrderRaw(requestParameters: StoreApiPlaceOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling placeOrder().'
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/apis/UserApi.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/apis/UserApi.ts
index c0830aaf5bee..b18b7fd43c57 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/apis/UserApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/apis/UserApi.ts
@@ -62,7 +62,7 @@ export class UserApi extends runtime.BaseAPI {
      * Create user
      */
     async createUserRaw(requestParameters: UserApiCreateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUser().'
@@ -98,7 +98,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithArrayInputRaw(requestParameters: UserApiCreateUsersWithArrayInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithArrayInput().'
@@ -133,7 +133,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithListInputRaw(requestParameters: UserApiCreateUsersWithListInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithListInput().'
@@ -169,7 +169,7 @@ export class UserApi extends runtime.BaseAPI {
      * Delete user
      */
     async deleteUserRaw(requestParameters: UserApiDeleteUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling deleteUser().'
@@ -202,7 +202,7 @@ export class UserApi extends runtime.BaseAPI {
      * Get user by user name
      */
     async getUserByNameRaw(requestParameters: UserApiGetUserByNameRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<User>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling getUserByName().'
@@ -235,14 +235,14 @@ export class UserApi extends runtime.BaseAPI {
      * Logs user into the system
      */
     async loginUserRaw(requestParameters: UserApiLoginUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<string>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling loginUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] == null) {
             throw new runtime.RequiredError(
                 'password',
                 'Required parameter "password" was null or undefined when calling loginUser().'
@@ -251,11 +251,11 @@ export class UserApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] != null) {
             queryParameters['username'] = requestParameters['username'];
         }
 
-        if (runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] != null) {
             queryParameters['password'] = requestParameters['password'];
         }
 
@@ -313,14 +313,14 @@ export class UserApi extends runtime.BaseAPI {
      * Updated user
      */
     async updateUserRaw(requestParameters: UserApiUpdateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling updateUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updateUser().'
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Category.ts
index 001bc135bb96..19e40eaf4b86 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Category.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A category for a pet
  * @export
@@ -45,22 +45,19 @@ export function CategoryFromJSON(json: any): Category {
 }
 
 export function CategoryFromJSONTyped(json: any, ignoreDiscriminator: boolean): Category {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function CategoryToJSON(value?: Category | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/ModelApiResponse.ts
index a7fc63d81f1b..4496efe73f05 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/ModelApiResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Describes the result of uploading an image resource
  * @export
@@ -51,23 +51,20 @@ export function ModelApiResponseFromJSON(json: any): ModelApiResponse {
 }
 
 export function ModelApiResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): ModelApiResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'code': !exists(json, 'code') ? undefined : json['code'],
-        'type': !exists(json, 'type') ? undefined : json['type'],
-        'message': !exists(json, 'message') ? undefined : json['message'],
+        'code': json['code'] == null ? undefined : json['code'],
+        'type': json['type'] == null ? undefined : json['type'],
+        'message': json['message'] == null ? undefined : json['message'],
     };
 }
 
 export function ModelApiResponseToJSON(value?: ModelApiResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Order.ts
index 9e12fc242793..948cda435c8a 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Order.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * An order for a pets from the pet store
  * @export
@@ -81,33 +81,30 @@ export function OrderFromJSON(json: any): Order {
 }
 
 export function OrderFromJSONTyped(json: any, ignoreDiscriminator: boolean): Order {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'petId': !exists(json, 'petId') ? undefined : json['petId'],
-        'quantity': !exists(json, 'quantity') ? undefined : json['quantity'],
-        'shipDate': !exists(json, 'shipDate') ? undefined : (new Date(json['shipDate'])),
-        'status': !exists(json, 'status') ? undefined : json['status'],
-        'complete': !exists(json, 'complete') ? undefined : json['complete'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'petId': json['petId'] == null ? undefined : json['petId'],
+        'quantity': json['quantity'] == null ? undefined : json['quantity'],
+        'shipDate': json['shipDate'] == null ? undefined : (new Date(json['shipDate'])),
+        'status': json['status'] == null ? undefined : json['status'],
+        'complete': json['complete'] == null ? undefined : json['complete'],
     };
 }
 
 export function OrderToJSON(value?: Order | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': !exists(value, 'shipDate') ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
         'status': value['status'],
         'complete': value['complete'],
     };
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Pet.ts
index 8906602f6f4b..3c60d980598b 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Pet.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Category } from './Category';
 import {
     CategoryFromJSON,
@@ -96,26 +96,23 @@ export function PetFromJSON(json: any): Pet {
 }
 
 export function PetFromJSONTyped(json: any, ignoreDiscriminator: boolean): Pet {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'category': !exists(json, 'category') ? undefined : CategoryFromJSON(json['category']),
+        'id': json['id'] == null ? undefined : json['id'],
+        'category': json['category'] == null ? undefined : CategoryFromJSON(json['category']),
         'name': json['name'],
         'photoUrls': json['photoUrls'],
-        'tags': !exists(json, 'tags') ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
-        'status': !exists(json, 'status') ? undefined : json['status'],
+        'tags': json['tags'] == null ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
+        'status': json['status'] == null ? undefined : json['status'],
     };
 }
 
 export function PetToJSON(value?: Pet | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
@@ -123,7 +120,7 @@ export function PetToJSON(value?: Pet | null): any {
         'category': CategoryToJSON(value['category']),
         'name': value['name'],
         'photoUrls': value['photoUrls'],
-        'tags': !exists(value, 'tags') ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
+        'tags': value['tags'] == null ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
         'status': value['status'],
     };
 }
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Tag.ts
index 24a25f383dba..0a5bdbdde07c 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/Tag.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A tag for a pet
  * @export
@@ -45,22 +45,19 @@ export function TagFromJSON(json: any): Tag {
 }
 
 export function TagFromJSONTyped(json: any, ignoreDiscriminator: boolean): Tag {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function TagToJSON(value?: Tag | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/User.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/User.ts
index 00ab35b80563..3924497622f9 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/models/User.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A User who is purchasing from the pet store
  * @export
@@ -81,28 +81,25 @@ export function UserFromJSON(json: any): User {
 }
 
 export function UserFromJSONTyped(json: any, ignoreDiscriminator: boolean): User {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'username': !exists(json, 'username') ? undefined : json['username'],
-        'firstName': !exists(json, 'firstName') ? undefined : json['firstName'],
-        'lastName': !exists(json, 'lastName') ? undefined : json['lastName'],
-        'email': !exists(json, 'email') ? undefined : json['email'],
-        'password': !exists(json, 'password') ? undefined : json['password'],
-        'phone': !exists(json, 'phone') ? undefined : json['phone'],
-        'userStatus': !exists(json, 'userStatus') ? undefined : json['userStatus'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'username': json['username'] == null ? undefined : json['username'],
+        'firstName': json['firstName'] == null ? undefined : json['firstName'],
+        'lastName': json['lastName'] == null ? undefined : json['lastName'],
+        'email': json['email'] == null ? undefined : json['email'],
+        'password': json['password'] == null ? undefined : json['password'],
+        'phone': json['phone'] == null ? undefined : json['phone'],
+        'userStatus': json['userStatus'] == null ? undefined : json['userStatus'],
     };
 }
 
 export function UserToJSON(value?: User | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts
index 813bf0fb4dfe..2c4103a8908e 100644
--- a/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/prefix-parameter-interfaces/src/runtime.ts
@@ -310,11 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/BehaviorApi.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/BehaviorApi.ts
index 1f9c29198de2..e20073b50cb4 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/BehaviorApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/BehaviorApi.ts
@@ -42,7 +42,7 @@ export class BehaviorApi extends runtime.BaseAPI {
      * Get permissions for the behavior
      */
     async getBehaviorPermissionsRaw(requestParameters: GetBehaviorPermissionsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<GetBehaviorPermissionsResponse>> {
-        if (!runtime.exists(requestParameters, 'behaviorId')) {
+        if (requestParameters['behaviorId'] == null) {
             throw new runtime.RequiredError(
                 'behaviorId',
                 'Required parameter "behaviorId" was null or undefined when calling getBehaviorPermissions().'
@@ -75,7 +75,7 @@ export class BehaviorApi extends runtime.BaseAPI {
      * Get the type of behavior
      */
     async getBehaviorTypeRaw(requestParameters: GetBehaviorTypeRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<GetBehaviorTypeResponse>> {
-        if (!runtime.exists(requestParameters, 'behaviorId')) {
+        if (requestParameters['behaviorId'] == null) {
             throw new runtime.RequiredError(
                 'behaviorId',
                 'Required parameter "behaviorId" was null or undefined when calling getBehaviorType().'
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/PetApi.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/PetApi.ts
index 8aa504326391..a283bd472223 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/PetApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/PetApi.ts
@@ -100,7 +100,7 @@ export class PetApi extends runtime.BaseAPI {
      * Add a new pet to the store
      */
     async addPetRaw(requestParameters: AddPetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'dummyCat')) {
+        if (requestParameters['dummyCat'] == null) {
             throw new runtime.RequiredError(
                 'dummyCat',
                 'Required parameter "dummyCat" was null or undefined when calling addPet().'
@@ -140,7 +140,7 @@ export class PetApi extends runtime.BaseAPI {
      * Deletes a pet
      */
     async deletePetRaw(requestParameters: DeletePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling deletePet().'
@@ -151,7 +151,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'apiKey')) {
+        if (requestParameters['apiKey'] != null) {
             headerParameters['api_key'] = String(requestParameters['apiKey']);
         }
 
@@ -183,7 +183,7 @@ export class PetApi extends runtime.BaseAPI {
      * @deprecated
      */
     async findPetsByIdsRaw(requestParameters: FindPetsByIdsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'ids')) {
+        if (requestParameters['ids'] == null) {
             throw new runtime.RequiredError(
                 'ids',
                 'Required parameter "ids" was null or undefined when calling findPetsByIds().'
@@ -192,7 +192,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'ids')) {
+        if (requestParameters['ids'] != null) {
             queryParameters['ids'] = requestParameters['ids']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -228,7 +228,7 @@ export class PetApi extends runtime.BaseAPI {
      * Finds Pets by status
      */
     async findPetsByStatusRaw(requestParameters: FindPetsByStatusRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<FindPetsByStatusResponse>> {
-        if (!runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] == null) {
             throw new runtime.RequiredError(
                 'status',
                 'Required parameter "status" was null or undefined when calling findPetsByStatus().'
@@ -237,7 +237,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             queryParameters['status'] = requestParameters['status']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -273,7 +273,7 @@ export class PetApi extends runtime.BaseAPI {
      * @deprecated
      */
     async findPetsByTagsRaw(requestParameters: FindPetsByTagsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] == null) {
             throw new runtime.RequiredError(
                 'tags',
                 'Required parameter "tags" was null or undefined when calling findPetsByTags().'
@@ -282,7 +282,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] != null) {
             queryParameters['tags'] = requestParameters['tags']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -319,7 +319,7 @@ export class PetApi extends runtime.BaseAPI {
      * @deprecated
      */
     async findPetsByUserIdsRaw(requestParameters: FindPetsByUserIdsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<FindPetsByUserResponse>> {
-        if (!runtime.exists(requestParameters, 'ids')) {
+        if (requestParameters['ids'] == null) {
             throw new runtime.RequiredError(
                 'ids',
                 'Required parameter "ids" was null or undefined when calling findPetsByUserIds().'
@@ -328,7 +328,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'ids')) {
+        if (requestParameters['ids'] != null) {
             queryParameters['ids'] = requestParameters['ids']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -364,7 +364,7 @@ export class PetApi extends runtime.BaseAPI {
      * Find pet by ID
      */
     async getPetByIdRaw(requestParameters: GetPetByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Pet>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling getPetById().'
@@ -402,7 +402,7 @@ export class PetApi extends runtime.BaseAPI {
      * Gets regions for a single pet.
      */
     async getPetRegionsRaw(requestParameters: GetPetRegionsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<PetRegionsResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling getPetRegions().'
@@ -435,7 +435,7 @@ export class PetApi extends runtime.BaseAPI {
      * Update an existing pet
      */
     async updatePetRaw(requestParameters: UpdatePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updatePet().'
@@ -475,14 +475,14 @@ export class PetApi extends runtime.BaseAPI {
      * Updates the pet regions.
      */
     async updatePetRegionsRaw(requestParameters: UpdatePetRegionsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<PetRegionsResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling updatePetRegions().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'newRegions')) {
+        if (requestParameters['newRegions'] == null) {
             throw new runtime.RequiredError(
                 'newRegions',
                 'Required parameter "newRegions" was null or undefined when calling updatePetRegions().'
@@ -518,7 +518,7 @@ export class PetApi extends runtime.BaseAPI {
      * Updates a pet in the store with form data
      */
     async updatePetWithFormRaw(requestParameters: UpdatePetWithFormRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling updatePetWithForm().'
@@ -548,11 +548,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'name')) {
+        if (requestParameters['name'] != null) {
             formParams.append('name', requestParameters['name'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             formParams.append('status', requestParameters['status'] as any);
         }
 
@@ -578,7 +578,7 @@ export class PetApi extends runtime.BaseAPI {
      * uploads an image
      */
     async uploadFileRaw(requestParameters: UploadFileRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<ModelApiResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling uploadFile().'
@@ -610,11 +610,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'additionalMetadata')) {
+        if (requestParameters['additionalMetadata'] != null) {
             formParams.append('additionalMetadata', requestParameters['additionalMetadata'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'file')) {
+        if (requestParameters['file'] != null) {
             formParams.append('file', requestParameters['file'] as any);
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/PetPartApi.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/PetPartApi.ts
index 5317ee8f514d..c1c514c3f8a3 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/PetPartApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/PetPartApi.ts
@@ -47,7 +47,7 @@ export class PetPartApi extends runtime.BaseAPI {
      * Returns single pet part type for the petPart id.
      */
     async getFakePetPartTypeRaw(requestParameters: GetFakePetPartTypeRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<GetPetPartTypeResponse>> {
-        if (!runtime.exists(requestParameters, 'fakePetPartId')) {
+        if (requestParameters['fakePetPartId'] == null) {
             throw new runtime.RequiredError(
                 'fakePetPartId',
                 'Required parameter "fakePetPartId" was null or undefined when calling getFakePetPartType().'
@@ -80,28 +80,28 @@ export class PetPartApi extends runtime.BaseAPI {
      * Get the matching parts for the given pet part.
      */
     async getMatchingPartsRaw(requestParameters: GetMatchingPartsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<GetMatchingPartsResponse>> {
-        if (!runtime.exists(requestParameters, 'fakePetPartId')) {
+        if (requestParameters['fakePetPartId'] == null) {
             throw new runtime.RequiredError(
                 'fakePetPartId',
                 'Required parameter "fakePetPartId" was null or undefined when calling getMatchingParts().'
             );
         }
 
-        if (!runtime.exists(requestParameters, '_long')) {
+        if (requestParameters['_long'] == null) {
             throw new runtime.RequiredError(
                 '_long',
                 'Required parameter "_long" was null or undefined when calling getMatchingParts().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'smooth')) {
+        if (requestParameters['smooth'] == null) {
             throw new runtime.RequiredError(
                 'smooth',
                 'Required parameter "smooth" was null or undefined when calling getMatchingParts().'
             );
         }
 
-        if (!runtime.exists(requestParameters, '_short')) {
+        if (requestParameters['_short'] == null) {
             throw new runtime.RequiredError(
                 '_short',
                 'Required parameter "_short" was null or undefined when calling getMatchingParts().'
@@ -110,23 +110,23 @@ export class PetPartApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, '_long')) {
+        if (requestParameters['_long'] != null) {
             queryParameters['long'] = requestParameters['_long'];
         }
 
-        if (runtime.exists(requestParameters, 'smooth')) {
+        if (requestParameters['smooth'] != null) {
             queryParameters['smooth'] = requestParameters['smooth'];
         }
 
-        if (runtime.exists(requestParameters, 'name')) {
+        if (requestParameters['name'] != null) {
             queryParameters['name'] = requestParameters['name'];
         }
 
-        if (runtime.exists(requestParameters, 'connectedPart')) {
+        if (requestParameters['connectedPart'] != null) {
             queryParameters['connected-part'] = requestParameters['connectedPart'];
         }
 
-        if (runtime.exists(requestParameters, '_short')) {
+        if (requestParameters['_short'] != null) {
             queryParameters['short'] = requestParameters['_short'];
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/StoreApi.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/StoreApi.ts
index 71f08a8daed8..8cf46da17dd8 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/StoreApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/StoreApi.ts
@@ -44,7 +44,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Delete purchase order by ID
      */
     async deleteOrderRaw(requestParameters: DeleteOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling deleteOrder().'
@@ -110,7 +110,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Find purchase order by ID
      */
     async getOrderByIdRaw(requestParameters: GetOrderByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling getOrderById().'
@@ -144,7 +144,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Place an order for a pet
      */
     async placeOrderRaw(requestParameters: PlaceOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling placeOrder().'
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/UserApi.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/UserApi.ts
index e2c54ff475c7..0a634a21cf2e 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/UserApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/apis/UserApi.ts
@@ -65,7 +65,7 @@ export class UserApi extends runtime.BaseAPI {
      * Create user
      */
     async createUserRaw(requestParameters: CreateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUser().'
@@ -101,7 +101,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithArrayInputRaw(requestParameters: CreateUsersWithArrayInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithArrayInput().'
@@ -136,7 +136,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithListInputRaw(requestParameters: CreateUsersWithListInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithListInput().'
@@ -172,7 +172,7 @@ export class UserApi extends runtime.BaseAPI {
      * Delete user
      */
     async deleteUserRaw(requestParameters: DeleteUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling deleteUser().'
@@ -205,7 +205,7 @@ export class UserApi extends runtime.BaseAPI {
      * Get user by user name
      */
     async getUserByNameRaw(requestParameters: GetUserByNameRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<User>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling getUserByName().'
@@ -238,14 +238,14 @@ export class UserApi extends runtime.BaseAPI {
      * Logs user into the system
      */
     async loginUserRaw(requestParameters: LoginUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<string>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling loginUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] == null) {
             throw new runtime.RequiredError(
                 'password',
                 'Required parameter "password" was null or undefined when calling loginUser().'
@@ -254,11 +254,11 @@ export class UserApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] != null) {
             queryParameters['username'] = requestParameters['username'];
         }
 
-        if (runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] != null) {
             queryParameters['password'] = requestParameters['password'];
         }
 
@@ -316,14 +316,14 @@ export class UserApi extends runtime.BaseAPI {
      * Updated user
      */
     async updateUserRaw(requestParameters: UpdateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<DefaultMetaOnlyResponse>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling updateUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updateUser().'
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Category.ts
index 001bc135bb96..19e40eaf4b86 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Category.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A category for a pet
  * @export
@@ -45,22 +45,19 @@ export function CategoryFromJSON(json: any): Category {
 }
 
 export function CategoryFromJSONTyped(json: any, ignoreDiscriminator: boolean): Category {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function CategoryToJSON(value?: Category | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/DefaultMetaOnlyResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/DefaultMetaOnlyResponse.ts
index 93b4baacc1a0..72d0569f4ccd 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/DefaultMetaOnlyResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/DefaultMetaOnlyResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { ResponseMeta } from './ResponseMeta';
 import {
     ResponseMetaFromJSON,
@@ -47,7 +47,7 @@ export function DefaultMetaOnlyResponseFromJSON(json: any): DefaultMetaOnlyRespo
 }
 
 export function DefaultMetaOnlyResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): DefaultMetaOnlyResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
@@ -57,11 +57,8 @@ export function DefaultMetaOnlyResponseFromJSONTyped(json: any, ignoreDiscrimina
 }
 
 export function DefaultMetaOnlyResponseToJSON(value?: DefaultMetaOnlyResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByStatusResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByStatusResponse.ts
index 2ee087651466..358b81b7f084 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByStatusResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByStatusResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Pet } from './Pet';
 import {
     PetFromJSON,
@@ -59,27 +59,24 @@ export function FindPetsByStatusResponseFromJSON(json: any): FindPetsByStatusRes
 }
 
 export function FindPetsByStatusResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): FindPetsByStatusResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
         'meta': ResponseMetaFromJSON(json['meta']),
-        'data': !exists(json, 'data') ? undefined : ((json['data'] as Array<any>).map(PetFromJSON)),
+        'data': json['data'] == null ? undefined : ((json['data'] as Array<any>).map(PetFromJSON)),
     };
 }
 
 export function FindPetsByStatusResponseToJSON(value?: FindPetsByStatusResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
         'meta': ResponseMetaToJSON(value['meta']),
-        'data': !exists(value, 'data') ? undefined : ((value['data'] as Array<any>).map(PetToJSON)),
+        'data': value['data'] == null ? undefined : ((value['data'] as Array<any>).map(PetToJSON)),
     };
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByUserResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByUserResponse.ts
index 49f4d6f17560..bfbe7a34cd95 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByUserResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/FindPetsByUserResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { ResponseMeta } from './ResponseMeta';
 import {
     ResponseMetaFromJSON,
@@ -59,27 +59,24 @@ export function FindPetsByUserResponseFromJSON(json: any): FindPetsByUserRespons
 }
 
 export function FindPetsByUserResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): FindPetsByUserResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
         'meta': ResponseMetaFromJSON(json['meta']),
-        'data': !exists(json, 'data') ? undefined : ((json['data'] as Array<any>).map(UserFromJSON)),
+        'data': json['data'] == null ? undefined : ((json['data'] as Array<any>).map(UserFromJSON)),
     };
 }
 
 export function FindPetsByUserResponseToJSON(value?: FindPetsByUserResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
         'meta': ResponseMetaToJSON(value['meta']),
-        'data': !exists(value, 'data') ? undefined : ((value['data'] as Array<any>).map(UserToJSON)),
+        'data': value['data'] == null ? undefined : ((value['data'] as Array<any>).map(UserToJSON)),
     };
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorPermissionsResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorPermissionsResponse.ts
index 10e2de71adc1..e25682be1f59 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorPermissionsResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorPermissionsResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { ResponseMeta } from './ResponseMeta';
 import {
     ResponseMetaFromJSON,
@@ -53,22 +53,19 @@ export function GetBehaviorPermissionsResponseFromJSON(json: any): GetBehaviorPe
 }
 
 export function GetBehaviorPermissionsResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): GetBehaviorPermissionsResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
         'meta': ResponseMetaFromJSON(json['meta']),
-        'data': !exists(json, 'data') ? undefined : json['data'],
+        'data': json['data'] == null ? undefined : json['data'],
     };
 }
 
 export function GetBehaviorPermissionsResponseToJSON(value?: GetBehaviorPermissionsResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorTypeResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorTypeResponse.ts
index d280962cf133..4f00ef278937 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorTypeResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetBehaviorTypeResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { BehaviorType } from './BehaviorType';
 import {
     BehaviorTypeFromJSON,
@@ -59,22 +59,19 @@ export function GetBehaviorTypeResponseFromJSON(json: any): GetBehaviorTypeRespo
 }
 
 export function GetBehaviorTypeResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): GetBehaviorTypeResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
         'meta': ResponseMetaFromJSON(json['meta']),
-        'data': !exists(json, 'data') ? undefined : BehaviorTypeFromJSON(json['data']),
+        'data': json['data'] == null ? undefined : BehaviorTypeFromJSON(json['data']),
     };
 }
 
 export function GetBehaviorTypeResponseToJSON(value?: GetBehaviorTypeResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetMatchingPartsResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetMatchingPartsResponse.ts
index 853ac803fd01..1db258479c3a 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetMatchingPartsResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetMatchingPartsResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { MatchingParts } from './MatchingParts';
 import {
     MatchingPartsFromJSON,
@@ -59,22 +59,19 @@ export function GetMatchingPartsResponseFromJSON(json: any): GetMatchingPartsRes
 }
 
 export function GetMatchingPartsResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): GetMatchingPartsResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
         'meta': ResponseMetaFromJSON(json['meta']),
-        'data': !exists(json, 'data') ? undefined : MatchingPartsFromJSON(json['data']),
+        'data': json['data'] == null ? undefined : MatchingPartsFromJSON(json['data']),
     };
 }
 
 export function GetMatchingPartsResponseToJSON(value?: GetMatchingPartsResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetPetPartTypeResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetPetPartTypeResponse.ts
index b39830b4601e..44815da1ca75 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetPetPartTypeResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/GetPetPartTypeResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { PetPartType } from './PetPartType';
 import {
     PetPartTypeFromJSON,
@@ -59,22 +59,19 @@ export function GetPetPartTypeResponseFromJSON(json: any): GetPetPartTypeRespons
 }
 
 export function GetPetPartTypeResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): GetPetPartTypeResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
         'meta': ResponseMetaFromJSON(json['meta']),
-        'data': !exists(json, 'data') ? undefined : PetPartTypeFromJSON(json['data']),
+        'data': json['data'] == null ? undefined : PetPartTypeFromJSON(json['data']),
     };
 }
 
 export function GetPetPartTypeResponseToJSON(value?: GetPetPartTypeResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ItemId.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ItemId.ts
index 69621e5eb7d7..5c2c26352428 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ItemId.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ItemId.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Simplified identifier of an item
  * @export
@@ -47,7 +47,7 @@ export function ItemIdFromJSON(json: any): ItemId {
 }
 
 export function ItemIdFromJSONTyped(json: any, ignoreDiscriminator: boolean): ItemId {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
@@ -58,11 +58,8 @@ export function ItemIdFromJSONTyped(json: any, ignoreDiscriminator: boolean): It
 }
 
 export function ItemIdToJSON(value?: ItemId | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/MatchingParts.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/MatchingParts.ts
index 125d78169bef..6c27dd2e8019 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/MatchingParts.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/MatchingParts.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Part } from './Part';
 import {
     PartFromJSON,
@@ -54,7 +54,7 @@ export function MatchingPartsFromJSON(json: any): MatchingParts {
 }
 
 export function MatchingPartsFromJSONTyped(json: any, ignoreDiscriminator: boolean): MatchingParts {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
@@ -65,11 +65,8 @@ export function MatchingPartsFromJSONTyped(json: any, ignoreDiscriminator: boole
 }
 
 export function MatchingPartsToJSON(value?: MatchingParts | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelApiResponse.ts
index a7fc63d81f1b..4496efe73f05 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelApiResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Describes the result of uploading an image resource
  * @export
@@ -51,23 +51,20 @@ export function ModelApiResponseFromJSON(json: any): ModelApiResponse {
 }
 
 export function ModelApiResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): ModelApiResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'code': !exists(json, 'code') ? undefined : json['code'],
-        'type': !exists(json, 'type') ? undefined : json['type'],
-        'message': !exists(json, 'message') ? undefined : json['message'],
+        'code': json['code'] == null ? undefined : json['code'],
+        'type': json['type'] == null ? undefined : json['type'],
+        'message': json['message'] == null ? undefined : json['message'],
     };
 }
 
 export function ModelApiResponseToJSON(value?: ModelApiResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelError.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelError.ts
index 166306af9663..75714ef92cce 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelError.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ModelError.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { ItemId } from './ItemId';
 import {
     ItemIdFromJSON,
@@ -65,24 +65,21 @@ export function ModelErrorFromJSON(json: any): ModelError {
 }
 
 export function ModelErrorFromJSONTyped(json: any, ignoreDiscriminator: boolean): ModelError {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
         'type': json['type'],
-        'itemInfo': !exists(json, 'itemInfo') ? undefined : ItemIdFromJSON(json['itemInfo']),
-        'details': !exists(json, 'details') ? undefined : json['details'],
-        'exception': !exists(json, 'exception') ? undefined : json['exception'],
+        'itemInfo': json['itemInfo'] == null ? undefined : ItemIdFromJSON(json['itemInfo']),
+        'details': json['details'] == null ? undefined : json['details'],
+        'exception': json['exception'] == null ? undefined : json['exception'],
     };
 }
 
 export function ModelErrorToJSON(value?: ModelError | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Order.ts
index 9e12fc242793..948cda435c8a 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Order.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * An order for a pets from the pet store
  * @export
@@ -81,33 +81,30 @@ export function OrderFromJSON(json: any): Order {
 }
 
 export function OrderFromJSONTyped(json: any, ignoreDiscriminator: boolean): Order {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'petId': !exists(json, 'petId') ? undefined : json['petId'],
-        'quantity': !exists(json, 'quantity') ? undefined : json['quantity'],
-        'shipDate': !exists(json, 'shipDate') ? undefined : (new Date(json['shipDate'])),
-        'status': !exists(json, 'status') ? undefined : json['status'],
-        'complete': !exists(json, 'complete') ? undefined : json['complete'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'petId': json['petId'] == null ? undefined : json['petId'],
+        'quantity': json['quantity'] == null ? undefined : json['quantity'],
+        'shipDate': json['shipDate'] == null ? undefined : (new Date(json['shipDate'])),
+        'status': json['status'] == null ? undefined : json['status'],
+        'complete': json['complete'] == null ? undefined : json['complete'],
     };
 }
 
 export function OrderToJSON(value?: Order | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': !exists(value, 'shipDate') ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
         'status': value['status'],
         'complete': value['complete'],
     };
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Part.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Part.ts
index 15f467e89f82..9d7ae4222138 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Part.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Part.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Contains all the info about a pet part
  * @export
@@ -47,7 +47,7 @@ export function PartFromJSON(json: any): Part {
 }
 
 export function PartFromJSONTyped(json: any, ignoreDiscriminator: boolean): Part {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
@@ -58,11 +58,8 @@ export function PartFromJSONTyped(json: any, ignoreDiscriminator: boolean): Part
 }
 
 export function PartToJSON(value?: Part | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Pet.ts
index c24da6bcb476..bc567b494366 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Pet.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Category } from './Category';
 import {
     CategoryFromJSON,
@@ -210,41 +210,38 @@ export function PetFromJSON(json: any): Pet {
 }
 
 export function PetFromJSONTyped(json: any, ignoreDiscriminator: boolean): Pet {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
         'id': json['id'],
-        'friendId': !exists(json, 'friendId') ? undefined : json['friendId'],
+        'friendId': json['friendId'] == null ? undefined : json['friendId'],
         'otherFriendIds': json['otherFriendIds'],
         'friendAge': json['friendAge'],
         'age': json['age'],
         'isHappy': json['isHappy'],
         'isTall': json['isTall'],
         'category': CategoryFromJSON(json['category']),
-        'optionalCategory': !exists(json, 'optionalCategory') ? undefined : CategoryFromJSON(json['optionalCategory']),
+        'optionalCategory': json['optionalCategory'] == null ? undefined : CategoryFromJSON(json['optionalCategory']),
         'name': json['name'],
-        '_entries': !exists(json, 'entries') ? undefined : ((json['entries'] as Array<any>).map(CategoryFromJSON)),
-        'surname': !exists(json, 'surname') ? undefined : json['surname'],
+        '_entries': json['entries'] == null ? undefined : ((json['entries'] as Array<any>).map(CategoryFromJSON)),
+        'surname': json['surname'] == null ? undefined : json['surname'],
         'photoUrls': json['photoUrls'],
         'warningStatus': WarningCodeFromJSON(json['warningStatus']),
-        'depStatus': !exists(json, 'depStatus') ? undefined : DeploymentRequestStatusFromJSON(json['depStatus']),
+        'depStatus': json['depStatus'] == null ? undefined : DeploymentRequestStatusFromJSON(json['depStatus']),
         'alternateStatus': DeploymentRequestStatusFromJSON(json['alternateStatus']),
         'otherDepStatuses': ((json['otherDepStatuses'] as Array<any>).map(DeploymentRequestStatusFromJSON)),
         'tags': ((json['tags'] as Array<any>).map(TagFromJSON)),
-        'optionalTags': !exists(json, 'optionalTags') ? undefined : ((json['optionalTags'] as Array<any>).map(TagFromJSON)),
+        'optionalTags': json['optionalTags'] == null ? undefined : ((json['optionalTags'] as Array<any>).map(TagFromJSON)),
         'status': json['status'],
-        'regions': !exists(json, 'regions') ? undefined : json['regions'],
+        'regions': json['regions'] == null ? undefined : json['regions'],
     };
 }
 
 export function PetToJSON(value?: Pet | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
@@ -258,7 +255,7 @@ export function PetToJSON(value?: Pet | null): any {
         'category': CategoryToJSON(value['category']),
         'optionalCategory': CategoryToJSON(value['optionalCategory']),
         'name': value['name'],
-        'entries': !exists(value, '_entries') ? undefined : ((value['_entries'] as Array<any>).map(CategoryToJSON)),
+        'entries': value['_entries'] == null ? undefined : ((value['_entries'] as Array<any>).map(CategoryToJSON)),
         'surname': value['surname'],
         'photoUrls': value['photoUrls'],
         'warningStatus': WarningCodeToJSON(value['warningStatus']),
@@ -266,7 +263,7 @@ export function PetToJSON(value?: Pet | null): any {
         'alternateStatus': DeploymentRequestStatusToJSON(value['alternateStatus']),
         'otherDepStatuses': ((value['otherDepStatuses'] as Array<any>).map(DeploymentRequestStatusToJSON)),
         'tags': ((value['tags'] as Array<any>).map(TagToJSON)),
-        'optionalTags': !exists(value, 'optionalTags') ? undefined : ((value['optionalTags'] as Array<any>).map(TagToJSON)),
+        'optionalTags': value['optionalTags'] == null ? undefined : ((value['optionalTags'] as Array<any>).map(TagToJSON)),
         'status': value['status'],
         'regions': value['regions'],
     };
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/PetRegionsResponse.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/PetRegionsResponse.ts
index cccb23bc1c54..400a5fa83e8d 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/PetRegionsResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/PetRegionsResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { ResponseMeta } from './ResponseMeta';
 import {
     ResponseMetaFromJSON,
@@ -53,22 +53,19 @@ export function PetRegionsResponseFromJSON(json: any): PetRegionsResponse {
 }
 
 export function PetRegionsResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): PetRegionsResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
         'meta': ResponseMetaFromJSON(json['meta']),
-        'data': !exists(json, 'data') ? undefined : json['data'],
+        'data': json['data'] == null ? undefined : json['data'],
     };
 }
 
 export function PetRegionsResponseToJSON(value?: PetRegionsResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ResponseMeta.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ResponseMeta.ts
index b0049adc685f..6b17720e996a 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ResponseMeta.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/ResponseMeta.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { ErrorCode } from './ErrorCode';
 import {
     ErrorCodeFromJSON,
@@ -109,26 +109,23 @@ export function ResponseMetaFromJSON(json: any): ResponseMeta {
 }
 
 export function ResponseMetaFromJSONTyped(json: any, ignoreDiscriminator: boolean): ResponseMeta {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
         'code': json['code'],
-        'detail': !exists(json, 'detail') ? undefined : json['detail'],
-        'exception': !exists(json, 'exception') ? undefined : json['exception'],
-        'type': !exists(json, 'type') ? undefined : json['type'],
-        'errorCode': !exists(json, 'errorCode') ? undefined : ErrorCodeFromJSON(json['errorCode']),
-        'errors': !exists(json, 'errors') ? undefined : json['errors'],
+        'detail': json['detail'] == null ? undefined : json['detail'],
+        'exception': json['exception'] == null ? undefined : json['exception'],
+        'type': json['type'] == null ? undefined : json['type'],
+        'errorCode': json['errorCode'] == null ? undefined : ErrorCodeFromJSON(json['errorCode']),
+        'errors': json['errors'] == null ? undefined : json['errors'],
     };
 }
 
 export function ResponseMetaToJSON(value?: ResponseMeta | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Tag.ts
index 24a25f383dba..0a5bdbdde07c 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/Tag.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A tag for a pet
  * @export
@@ -45,22 +45,19 @@ export function TagFromJSON(json: any): Tag {
 }
 
 export function TagFromJSONTyped(json: any, ignoreDiscriminator: boolean): Tag {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function TagToJSON(value?: Tag | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/User.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/User.ts
index 9f4c7d4d4c17..c6cb145be14e 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/models/User.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A User who is purchasing from the pet store
  * @export
@@ -95,30 +95,27 @@ export function UserFromJSON(json: any): User {
 }
 
 export function UserFromJSONTyped(json: any, ignoreDiscriminator: boolean): User {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
         'id': json['id'],
-        'username': !exists(json, 'username') ? undefined : json['username'],
-        'firstName': !exists(json, 'firstName') ? undefined : json['firstName'],
-        'lastName': !exists(json, 'lastName') ? undefined : json['lastName'],
-        'email': !exists(json, 'email') ? undefined : json['email'],
-        'password': !exists(json, 'password') ? undefined : json['password'],
-        'phone': !exists(json, 'phone') ? undefined : json['phone'],
-        'userStatus': !exists(json, 'userStatus') ? undefined : json['userStatus'],
-        'subUser': !exists(json, 'subUser') ? undefined : UserFromJSON(json['subUser']),
+        'username': json['username'] == null ? undefined : json['username'],
+        'firstName': json['firstName'] == null ? undefined : json['firstName'],
+        'lastName': json['lastName'] == null ? undefined : json['lastName'],
+        'email': json['email'] == null ? undefined : json['email'],
+        'password': json['password'] == null ? undefined : json['password'],
+        'phone': json['phone'] == null ? undefined : json['phone'],
+        'userStatus': json['userStatus'] == null ? undefined : json['userStatus'],
+        'subUser': json['subUser'] == null ? undefined : UserFromJSON(json['subUser']),
         'subUser2': UserFromJSON(json['subUser2']),
     };
 }
 
 export function UserToJSON(value?: User | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts
index 813bf0fb4dfe..2c4103a8908e 100644
--- a/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/sagas-and-records/src/runtime.ts
@@ -310,11 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/AnotherFakeApi.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/AnotherFakeApi.ts
index cec7e569d020..e63d48d42bd6 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/AnotherFakeApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/AnotherFakeApi.ts
@@ -36,7 +36,7 @@ export class AnotherFakeApi extends runtime.BaseAPI {
      * To test special tags
      */
     async _123testSpecialTagsRaw(requestParameters: 123testSpecialTagsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Client>> {
-        if (!runtime.exists(requestParameters, 'client')) {
+        if (requestParameters['client'] == null) {
             throw new runtime.RequiredError(
                 'client',
                 'Required parameter "client" was null or undefined when calling _123testSpecialTags().'
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/FakeApi.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/FakeApi.ts
index db35c115a6ff..dd1a525ac391 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/FakeApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/FakeApi.ts
@@ -207,7 +207,7 @@ export class FakeApi extends runtime.BaseAPI {
      * test http signature authentication
      */
     async fakeHttpSignatureTestRaw(requestParameters: FakeHttpSignatureTestRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'pet')) {
+        if (requestParameters['pet'] == null) {
             throw new runtime.RequiredError(
                 'pet',
                 'Required parameter "pet" was null or undefined when calling fakeHttpSignatureTest().'
@@ -216,7 +216,7 @@ export class FakeApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'query1')) {
+        if (requestParameters['query1'] != null) {
             queryParameters['query_1'] = requestParameters['query1'];
         }
 
@@ -224,7 +224,7 @@ export class FakeApi extends runtime.BaseAPI {
 
         headerParameters['Content-Type'] = 'application/json';
 
-        if (runtime.exists(requestParameters, 'header1')) {
+        if (requestParameters['header1'] != null) {
             headerParameters['header_1'] = String(requestParameters['header1']);
         }
 
@@ -378,7 +378,7 @@ export class FakeApi extends runtime.BaseAPI {
      * Test serialization of enum (int) properties with examples
      */
     async fakePropertyEnumIntegerSerializeRaw(requestParameters: FakePropertyEnumIntegerSerializeRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<OuterObjectWithEnumProperty>> {
-        if (!runtime.exists(requestParameters, 'outerObjectWithEnumProperty')) {
+        if (requestParameters['outerObjectWithEnumProperty'] == null) {
             throw new runtime.RequiredError(
                 'outerObjectWithEnumProperty',
                 'Required parameter "outerObjectWithEnumProperty" was null or undefined when calling fakePropertyEnumIntegerSerialize().'
@@ -414,7 +414,7 @@ export class FakeApi extends runtime.BaseAPI {
      * For this test, the body has to be a binary file.
      */
     async testBodyWithBinaryRaw(requestParameters: TestBodyWithBinaryRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling testBodyWithBinary().'
@@ -449,7 +449,7 @@ export class FakeApi extends runtime.BaseAPI {
      * For this test, the body for this request must reference a schema named `File`.
      */
     async testBodyWithFileSchemaRaw(requestParameters: TestBodyWithFileSchemaRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'fileSchemaTestClass')) {
+        if (requestParameters['fileSchemaTestClass'] == null) {
             throw new runtime.RequiredError(
                 'fileSchemaTestClass',
                 'Required parameter "fileSchemaTestClass" was null or undefined when calling testBodyWithFileSchema().'
@@ -483,14 +483,14 @@ export class FakeApi extends runtime.BaseAPI {
     /**
      */
     async testBodyWithQueryParamsRaw(requestParameters: TestBodyWithQueryParamsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'query')) {
+        if (requestParameters['query'] == null) {
             throw new runtime.RequiredError(
                 'query',
                 'Required parameter "query" was null or undefined when calling testBodyWithQueryParams().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'user')) {
+        if (requestParameters['user'] == null) {
             throw new runtime.RequiredError(
                 'user',
                 'Required parameter "user" was null or undefined when calling testBodyWithQueryParams().'
@@ -499,7 +499,7 @@ export class FakeApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'query')) {
+        if (requestParameters['query'] != null) {
             queryParameters['query'] = requestParameters['query'];
         }
 
@@ -529,7 +529,7 @@ export class FakeApi extends runtime.BaseAPI {
      * To test \"client\" model
      */
     async testClientModelRaw(requestParameters: TestClientModelRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Client>> {
-        if (!runtime.exists(requestParameters, 'client')) {
+        if (requestParameters['client'] == null) {
             throw new runtime.RequiredError(
                 'client',
                 'Required parameter "client" was null or undefined when calling testClientModel().'
@@ -567,28 +567,28 @@ export class FakeApi extends runtime.BaseAPI {
      * Fake endpoint for testing various parameters 假端點 偽のエンドポイント 가짜 엔드 포인트 
      */
     async testEndpointParametersRaw(requestParameters: TestEndpointParametersRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'number')) {
+        if (requestParameters['number'] == null) {
             throw new runtime.RequiredError(
                 'number',
                 'Required parameter "number" was null or undefined when calling testEndpointParameters().'
             );
         }
 
-        if (!runtime.exists(requestParameters, '_double')) {
+        if (requestParameters['_double'] == null) {
             throw new runtime.RequiredError(
                 '_double',
                 'Required parameter "_double" was null or undefined when calling testEndpointParameters().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'patternWithoutDelimiter')) {
+        if (requestParameters['patternWithoutDelimiter'] == null) {
             throw new runtime.RequiredError(
                 'patternWithoutDelimiter',
                 'Required parameter "patternWithoutDelimiter" was null or undefined when calling testEndpointParameters().'
             );
         }
 
-        if (!runtime.exists(requestParameters, '_byte')) {
+        if (requestParameters['_byte'] == null) {
             throw new runtime.RequiredError(
                 '_byte',
                 'Required parameter "_byte" was null or undefined when calling testEndpointParameters().'
@@ -618,59 +618,59 @@ export class FakeApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'integer')) {
+        if (requestParameters['integer'] != null) {
             formParams.append('integer', requestParameters['integer'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'int32')) {
+        if (requestParameters['int32'] != null) {
             formParams.append('int32', requestParameters['int32'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'int64')) {
+        if (requestParameters['int64'] != null) {
             formParams.append('int64', requestParameters['int64'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'number')) {
+        if (requestParameters['number'] != null) {
             formParams.append('number', requestParameters['number'] as any);
         }
 
-        if (runtime.exists(requestParameters, '_float')) {
+        if (requestParameters['_float'] != null) {
             formParams.append('float', requestParameters['_float'] as any);
         }
 
-        if (runtime.exists(requestParameters, '_double')) {
+        if (requestParameters['_double'] != null) {
             formParams.append('double', requestParameters['_double'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'string')) {
+        if (requestParameters['string'] != null) {
             formParams.append('string', requestParameters['string'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'patternWithoutDelimiter')) {
+        if (requestParameters['patternWithoutDelimiter'] != null) {
             formParams.append('pattern_without_delimiter', requestParameters['patternWithoutDelimiter'] as any);
         }
 
-        if (runtime.exists(requestParameters, '_byte')) {
+        if (requestParameters['_byte'] != null) {
             formParams.append('byte', requestParameters['_byte'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'binary')) {
+        if (requestParameters['binary'] != null) {
             formParams.append('binary', requestParameters['binary'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'date')) {
+        if (requestParameters['date'] != null) {
             formParams.append('date', requestParameters['date'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'dateTime')) {
+        if (requestParameters['dateTime'] != null) {
             formParams.append('dateTime', requestParameters['dateTime'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] != null) {
             formParams.append('password', requestParameters['password'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'callback')) {
+        if (requestParameters['callback'] != null) {
             formParams.append('callback', requestParameters['callback'] as any);
         }
 
@@ -700,33 +700,33 @@ export class FakeApi extends runtime.BaseAPI {
     async testEnumParametersRaw(requestParameters: TestEnumParametersRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'enumQueryStringArray')) {
+        if (requestParameters['enumQueryStringArray'] != null) {
             queryParameters['enum_query_string_array'] = requestParameters['enumQueryStringArray'];
         }
 
-        if (runtime.exists(requestParameters, 'enumQueryString')) {
+        if (requestParameters['enumQueryString'] != null) {
             queryParameters['enum_query_string'] = requestParameters['enumQueryString'];
         }
 
-        if (runtime.exists(requestParameters, 'enumQueryInteger')) {
+        if (requestParameters['enumQueryInteger'] != null) {
             queryParameters['enum_query_integer'] = requestParameters['enumQueryInteger'];
         }
 
-        if (runtime.exists(requestParameters, 'enumQueryDouble')) {
+        if (requestParameters['enumQueryDouble'] != null) {
             queryParameters['enum_query_double'] = requestParameters['enumQueryDouble'];
         }
 
-        if (runtime.exists(requestParameters, 'enumQueryModelArray')) {
+        if (requestParameters['enumQueryModelArray'] != null) {
             queryParameters['enum_query_model_array'] = requestParameters['enumQueryModelArray'];
         }
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'enumHeaderStringArray')) {
+        if (requestParameters['enumHeaderStringArray'] != null) {
             headerParameters['enum_header_string_array'] = requestParameters['enumHeaderStringArray']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
-        if (runtime.exists(requestParameters, 'enumHeaderString')) {
+        if (requestParameters['enumHeaderString'] != null) {
             headerParameters['enum_header_string'] = String(requestParameters['enumHeaderString']);
         }
 
@@ -744,11 +744,11 @@ export class FakeApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'enumFormStringArray')) {
+        if (requestParameters['enumFormStringArray'] != null) {
             formParams.append('enum_form_string_array', requestParameters['enumFormStringArray']!.join(runtime.COLLECTION_FORMATS["csv"]));
         }
 
-        if (runtime.exists(requestParameters, 'enumFormString')) {
+        if (requestParameters['enumFormString'] != null) {
             formParams.append('enum_form_string', requestParameters['enumFormString'] as any);
         }
 
@@ -776,21 +776,21 @@ export class FakeApi extends runtime.BaseAPI {
      * Fake endpoint to test group parameters (optional)
      */
     async testGroupParametersRaw(requestParameters: TestGroupParametersRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'requiredStringGroup')) {
+        if (requestParameters['requiredStringGroup'] == null) {
             throw new runtime.RequiredError(
                 'requiredStringGroup',
                 'Required parameter "requiredStringGroup" was null or undefined when calling testGroupParameters().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'requiredBooleanGroup')) {
+        if (requestParameters['requiredBooleanGroup'] == null) {
             throw new runtime.RequiredError(
                 'requiredBooleanGroup',
                 'Required parameter "requiredBooleanGroup" was null or undefined when calling testGroupParameters().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'requiredInt64Group')) {
+        if (requestParameters['requiredInt64Group'] == null) {
             throw new runtime.RequiredError(
                 'requiredInt64Group',
                 'Required parameter "requiredInt64Group" was null or undefined when calling testGroupParameters().'
@@ -799,29 +799,29 @@ export class FakeApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'requiredStringGroup')) {
+        if (requestParameters['requiredStringGroup'] != null) {
             queryParameters['required_string_group'] = requestParameters['requiredStringGroup'];
         }
 
-        if (runtime.exists(requestParameters, 'requiredInt64Group')) {
+        if (requestParameters['requiredInt64Group'] != null) {
             queryParameters['required_int64_group'] = requestParameters['requiredInt64Group'];
         }
 
-        if (runtime.exists(requestParameters, 'stringGroup')) {
+        if (requestParameters['stringGroup'] != null) {
             queryParameters['string_group'] = requestParameters['stringGroup'];
         }
 
-        if (runtime.exists(requestParameters, 'int64Group')) {
+        if (requestParameters['int64Group'] != null) {
             queryParameters['int64_group'] = requestParameters['int64Group'];
         }
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'requiredBooleanGroup')) {
+        if (requestParameters['requiredBooleanGroup'] != null) {
             headerParameters['required_boolean_group'] = String(requestParameters['requiredBooleanGroup']);
         }
 
-        if (runtime.exists(requestParameters, 'booleanGroup')) {
+        if (requestParameters['booleanGroup'] != null) {
             headerParameters['boolean_group'] = String(requestParameters['booleanGroup']);
         }
 
@@ -856,7 +856,7 @@ export class FakeApi extends runtime.BaseAPI {
      * test inline additionalProperties
      */
     async testInlineAdditionalPropertiesRaw(requestParameters: TestInlineAdditionalPropertiesRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'requestBody')) {
+        if (requestParameters['requestBody'] == null) {
             throw new runtime.RequiredError(
                 'requestBody',
                 'Required parameter "requestBody" was null or undefined when calling testInlineAdditionalProperties().'
@@ -893,14 +893,14 @@ export class FakeApi extends runtime.BaseAPI {
      * test json serialization of form data
      */
     async testJsonFormDataRaw(requestParameters: TestJsonFormDataRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'param')) {
+        if (requestParameters['param'] == null) {
             throw new runtime.RequiredError(
                 'param',
                 'Required parameter "param" was null or undefined when calling testJsonFormData().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'param2')) {
+        if (requestParameters['param2'] == null) {
             throw new runtime.RequiredError(
                 'param2',
                 'Required parameter "param2" was null or undefined when calling testJsonFormData().'
@@ -925,11 +925,11 @@ export class FakeApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'param')) {
+        if (requestParameters['param'] != null) {
             formParams.append('param', requestParameters['param'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'param2')) {
+        if (requestParameters['param2'] != null) {
             formParams.append('param2', requestParameters['param2'] as any);
         }
 
@@ -956,42 +956,42 @@ export class FakeApi extends runtime.BaseAPI {
      * To test the collection format in query parameters
      */
     async testQueryParameterCollectionFormatRaw(requestParameters: TestQueryParameterCollectionFormatRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'pipe')) {
+        if (requestParameters['pipe'] == null) {
             throw new runtime.RequiredError(
                 'pipe',
                 'Required parameter "pipe" was null or undefined when calling testQueryParameterCollectionFormat().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'ioutil')) {
+        if (requestParameters['ioutil'] == null) {
             throw new runtime.RequiredError(
                 'ioutil',
                 'Required parameter "ioutil" was null or undefined when calling testQueryParameterCollectionFormat().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'http')) {
+        if (requestParameters['http'] == null) {
             throw new runtime.RequiredError(
                 'http',
                 'Required parameter "http" was null or undefined when calling testQueryParameterCollectionFormat().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'url')) {
+        if (requestParameters['url'] == null) {
             throw new runtime.RequiredError(
                 'url',
                 'Required parameter "url" was null or undefined when calling testQueryParameterCollectionFormat().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'context')) {
+        if (requestParameters['context'] == null) {
             throw new runtime.RequiredError(
                 'context',
                 'Required parameter "context" was null or undefined when calling testQueryParameterCollectionFormat().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'allowEmpty')) {
+        if (requestParameters['allowEmpty'] == null) {
             throw new runtime.RequiredError(
                 'allowEmpty',
                 'Required parameter "allowEmpty" was null or undefined when calling testQueryParameterCollectionFormat().'
@@ -1000,31 +1000,31 @@ export class FakeApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'pipe')) {
+        if (requestParameters['pipe'] != null) {
             queryParameters['pipe'] = requestParameters['pipe']!.join(runtime.COLLECTION_FORMATS["pipes"]);
         }
 
-        if (runtime.exists(requestParameters, 'ioutil')) {
+        if (requestParameters['ioutil'] != null) {
             queryParameters['ioutil'] = requestParameters['ioutil']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
-        if (runtime.exists(requestParameters, 'http')) {
+        if (requestParameters['http'] != null) {
             queryParameters['http'] = requestParameters['http']!.join(runtime.COLLECTION_FORMATS["ssv"]);
         }
 
-        if (runtime.exists(requestParameters, 'url')) {
+        if (requestParameters['url'] != null) {
             queryParameters['url'] = requestParameters['url']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
-        if (runtime.exists(requestParameters, 'context')) {
+        if (requestParameters['context'] != null) {
             queryParameters['context'] = requestParameters['context'];
         }
 
-        if (runtime.exists(requestParameters, 'language')) {
+        if (requestParameters['language'] != null) {
             queryParameters['language'] = requestParameters['language'];
         }
 
-        if (runtime.exists(requestParameters, 'allowEmpty')) {
+        if (requestParameters['allowEmpty'] != null) {
             queryParameters['allowEmpty'] = requestParameters['allowEmpty'];
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/FakeClassnameTags123Api.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/FakeClassnameTags123Api.ts
index ce9f567a5324..e2cf17eb7a26 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/FakeClassnameTags123Api.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/FakeClassnameTags123Api.ts
@@ -36,7 +36,7 @@ export class FakeClassnameTags123Api extends runtime.BaseAPI {
      * To test class name in snake case
      */
     async testClassnameRaw(requestParameters: TestClassnameRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Client>> {
-        if (!runtime.exists(requestParameters, 'client')) {
+        if (requestParameters['client'] == null) {
             throw new runtime.RequiredError(
                 'client',
                 'Required parameter "client" was null or undefined when calling testClassname().'
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/PetApi.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/PetApi.ts
index 389f7d7077f3..5b77f2de4486 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/PetApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/PetApi.ts
@@ -78,7 +78,7 @@ export class PetApi extends runtime.BaseAPI {
      * Add a new pet to the store
      */
     async addPetRaw(requestParameters: AddPetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'pet')) {
+        if (requestParameters['pet'] == null) {
             throw new runtime.RequiredError(
                 'pet',
                 'Required parameter "pet" was null or undefined when calling addPet().'
@@ -120,7 +120,7 @@ export class PetApi extends runtime.BaseAPI {
      * Deletes a pet
      */
     async deletePetRaw(requestParameters: DeletePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling deletePet().'
@@ -131,7 +131,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'apiKey')) {
+        if (requestParameters['apiKey'] != null) {
             headerParameters['api_key'] = String(requestParameters['apiKey']);
         }
 
@@ -163,7 +163,7 @@ export class PetApi extends runtime.BaseAPI {
      * Finds Pets by status
      */
     async findPetsByStatusRaw(requestParameters: FindPetsByStatusRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] == null) {
             throw new runtime.RequiredError(
                 'status',
                 'Required parameter "status" was null or undefined when calling findPetsByStatus().'
@@ -172,7 +172,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             queryParameters['status'] = requestParameters['status']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -208,7 +208,7 @@ export class PetApi extends runtime.BaseAPI {
      * @deprecated
      */
     async findPetsByTagsRaw(requestParameters: FindPetsByTagsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Set<Pet>>> {
-        if (!runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] == null) {
             throw new runtime.RequiredError(
                 'tags',
                 'Required parameter "tags" was null or undefined when calling findPetsByTags().'
@@ -217,7 +217,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] != null) {
             queryParameters['tags'] = Array.from(requestParameters['tags'])!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -253,7 +253,7 @@ export class PetApi extends runtime.BaseAPI {
      * Find pet by ID
      */
     async getPetByIdRaw(requestParameters: GetPetByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Pet>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling getPetById().'
@@ -292,7 +292,7 @@ export class PetApi extends runtime.BaseAPI {
      * Update an existing pet
      */
     async updatePetRaw(requestParameters: UpdatePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'pet')) {
+        if (requestParameters['pet'] == null) {
             throw new runtime.RequiredError(
                 'pet',
                 'Required parameter "pet" was null or undefined when calling updatePet().'
@@ -334,7 +334,7 @@ export class PetApi extends runtime.BaseAPI {
      * Updates a pet in the store with form data
      */
     async updatePetWithFormRaw(requestParameters: UpdatePetWithFormRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling updatePetWithForm().'
@@ -364,11 +364,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'name')) {
+        if (requestParameters['name'] != null) {
             formParams.append('name', requestParameters['name'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             formParams.append('status', requestParameters['status'] as any);
         }
 
@@ -396,7 +396,7 @@ export class PetApi extends runtime.BaseAPI {
      * uploads an image
      */
     async uploadFileRaw(requestParameters: UploadFileRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<ModelApiResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling uploadFile().'
@@ -428,11 +428,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'additionalMetadata')) {
+        if (requestParameters['additionalMetadata'] != null) {
             formParams.append('additionalMetadata', requestParameters['additionalMetadata'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'file')) {
+        if (requestParameters['file'] != null) {
             formParams.append('file', requestParameters['file'] as any);
         }
 
@@ -461,14 +461,14 @@ export class PetApi extends runtime.BaseAPI {
      * uploads an image (required)
      */
     async uploadFileWithRequiredFileRaw(requestParameters: UploadFileWithRequiredFileRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<ModelApiResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling uploadFileWithRequiredFile().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'requiredFile')) {
+        if (requestParameters['requiredFile'] == null) {
             throw new runtime.RequiredError(
                 'requiredFile',
                 'Required parameter "requiredFile" was null or undefined when calling uploadFileWithRequiredFile().'
@@ -500,11 +500,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'additionalMetadata')) {
+        if (requestParameters['additionalMetadata'] != null) {
             formParams.append('additionalMetadata', requestParameters['additionalMetadata'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'requiredFile')) {
+        if (requestParameters['requiredFile'] != null) {
             formParams.append('requiredFile', requestParameters['requiredFile'] as any);
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/StoreApi.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/StoreApi.ts
index 4444f71d4cd0..4a7ee687d8dc 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/StoreApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/StoreApi.ts
@@ -44,7 +44,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Delete purchase order by ID
      */
     async deleteOrderRaw(requestParameters: DeleteOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling deleteOrder().'
@@ -110,7 +110,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Find purchase order by ID
      */
     async getOrderByIdRaw(requestParameters: GetOrderByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling getOrderById().'
@@ -145,7 +145,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Place an order for a pet
      */
     async placeOrderRaw(requestParameters: PlaceOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'order')) {
+        if (requestParameters['order'] == null) {
             throw new runtime.RequiredError(
                 'order',
                 'Required parameter "order" was null or undefined when calling placeOrder().'
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/UserApi.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/UserApi.ts
index 36114480012f..b151cb1eaaca 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/UserApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/apis/UserApi.ts
@@ -62,7 +62,7 @@ export class UserApi extends runtime.BaseAPI {
      * Create user
      */
     async createUserRaw(requestParameters: CreateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'user')) {
+        if (requestParameters['user'] == null) {
             throw new runtime.RequiredError(
                 'user',
                 'Required parameter "user" was null or undefined when calling createUser().'
@@ -99,7 +99,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithArrayInputRaw(requestParameters: CreateUsersWithArrayInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'user')) {
+        if (requestParameters['user'] == null) {
             throw new runtime.RequiredError(
                 'user',
                 'Required parameter "user" was null or undefined when calling createUsersWithArrayInput().'
@@ -136,7 +136,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithListInputRaw(requestParameters: CreateUsersWithListInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'user')) {
+        if (requestParameters['user'] == null) {
             throw new runtime.RequiredError(
                 'user',
                 'Required parameter "user" was null or undefined when calling createUsersWithListInput().'
@@ -173,7 +173,7 @@ export class UserApi extends runtime.BaseAPI {
      * Delete user
      */
     async deleteUserRaw(requestParameters: DeleteUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling deleteUser().'
@@ -207,7 +207,7 @@ export class UserApi extends runtime.BaseAPI {
      * Get user by user name
      */
     async getUserByNameRaw(requestParameters: GetUserByNameRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<User>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling getUserByName().'
@@ -242,14 +242,14 @@ export class UserApi extends runtime.BaseAPI {
      * Logs user into the system
      */
     async loginUserRaw(requestParameters: LoginUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<string>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling loginUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] == null) {
             throw new runtime.RequiredError(
                 'password',
                 'Required parameter "password" was null or undefined when calling loginUser().'
@@ -258,11 +258,11 @@ export class UserApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] != null) {
             queryParameters['username'] = requestParameters['username'];
         }
 
-        if (runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] != null) {
             queryParameters['password'] = requestParameters['password'];
         }
 
@@ -323,14 +323,14 @@ export class UserApi extends runtime.BaseAPI {
      * Updated user
      */
     async updateUserRaw(requestParameters: UpdateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling updateUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'user')) {
+        if (requestParameters['user'] == null) {
             throw new runtime.RequiredError(
                 'user',
                 'Required parameter "user" was null or undefined when calling updateUser().'
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AdditionalPropertiesClass.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AdditionalPropertiesClass.ts
index 429a86538cd1..f0c934a942c6 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AdditionalPropertiesClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AdditionalPropertiesClass.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -45,22 +45,19 @@ export function AdditionalPropertiesClassFromJSON(json: any): AdditionalProperti
 }
 
 export function AdditionalPropertiesClassFromJSONTyped(json: any, ignoreDiscriminator: boolean): AdditionalPropertiesClass {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'mapProperty': !exists(json, 'map_property') ? undefined : json['map_property'],
-        'mapOfMapProperty': !exists(json, 'map_of_map_property') ? undefined : json['map_of_map_property'],
+        'mapProperty': json['map_property'] == null ? undefined : json['map_property'],
+        'mapOfMapProperty': json['map_of_map_property'] == null ? undefined : json['map_of_map_property'],
     };
 }
 
 export function AdditionalPropertiesClassToJSON(value?: AdditionalPropertiesClass | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AllOfWithSingleRef.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AllOfWithSingleRef.ts
index 5fc70179d079..8f9802529e4b 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AllOfWithSingleRef.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/AllOfWithSingleRef.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { SingleRefType } from './SingleRefType';
 import {
     SingleRefTypeFromJSON,
@@ -52,22 +52,19 @@ export function AllOfWithSingleRefFromJSON(json: any): AllOfWithSingleRef {
 }
 
 export function AllOfWithSingleRefFromJSONTyped(json: any, ignoreDiscriminator: boolean): AllOfWithSingleRef {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'username': !exists(json, 'username') ? undefined : json['username'],
-        'singleRefType': !exists(json, 'SingleRefType') ? undefined : SingleRefTypeFromJSON(json['SingleRefType']),
+        'username': json['username'] == null ? undefined : json['username'],
+        'singleRefType': json['SingleRefType'] == null ? undefined : SingleRefTypeFromJSON(json['SingleRefType']),
     };
 }
 
 export function AllOfWithSingleRefToJSON(value?: AllOfWithSingleRef | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts
index 30313f92a07b..c2394952c9de 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Animal.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import {
      CatFromJSONTyped,
      DogFromJSONTyped
@@ -51,7 +51,7 @@ export function AnimalFromJSON(json: any): Animal {
 }
 
 export function AnimalFromJSONTyped(json: any, ignoreDiscriminator: boolean): Animal {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     if (!ignoreDiscriminator) {
@@ -65,16 +65,13 @@ export function AnimalFromJSONTyped(json: any, ignoreDiscriminator: boolean): An
     return {
         
         'className': json['class_name'],
-        'color': !exists(json, 'color') ? undefined : json['color'],
+        'color': json['color'] == null ? undefined : json['color'],
     };
 }
 
 export function AnimalToJSON(value?: Animal | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfArrayOfNumberOnly.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfArrayOfNumberOnly.ts
index 6aca52358716..6e4c4ac139c3 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfArrayOfNumberOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfArrayOfNumberOnly.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function ArrayOfArrayOfNumberOnlyFromJSON(json: any): ArrayOfArrayOfNumbe
 }
 
 export function ArrayOfArrayOfNumberOnlyFromJSONTyped(json: any, ignoreDiscriminator: boolean): ArrayOfArrayOfNumberOnly {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'arrayArrayNumber': !exists(json, 'ArrayArrayNumber') ? undefined : json['ArrayArrayNumber'],
+        'arrayArrayNumber': json['ArrayArrayNumber'] == null ? undefined : json['ArrayArrayNumber'],
     };
 }
 
 export function ArrayOfArrayOfNumberOnlyToJSON(value?: ArrayOfArrayOfNumberOnly | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfNumberOnly.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfNumberOnly.ts
index b39a628ed121..4c0e2e98805b 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfNumberOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayOfNumberOnly.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function ArrayOfNumberOnlyFromJSON(json: any): ArrayOfNumberOnly {
 }
 
 export function ArrayOfNumberOnlyFromJSONTyped(json: any, ignoreDiscriminator: boolean): ArrayOfNumberOnly {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'arrayNumber': !exists(json, 'ArrayNumber') ? undefined : json['ArrayNumber'],
+        'arrayNumber': json['ArrayNumber'] == null ? undefined : json['ArrayNumber'],
     };
 }
 
 export function ArrayOfNumberOnlyToJSON(value?: ArrayOfNumberOnly | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayTest.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayTest.ts
index 2396708971fa..2cdb8f33fa6f 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ArrayTest.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { ReadOnlyFirst } from './ReadOnlyFirst';
 import {
     ReadOnlyFirstFromJSON,
@@ -58,23 +58,20 @@ export function ArrayTestFromJSON(json: any): ArrayTest {
 }
 
 export function ArrayTestFromJSONTyped(json: any, ignoreDiscriminator: boolean): ArrayTest {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'arrayOfString': !exists(json, 'array_of_string') ? undefined : json['array_of_string'],
-        'arrayArrayOfInteger': !exists(json, 'array_array_of_integer') ? undefined : json['array_array_of_integer'],
-        'arrayArrayOfModel': !exists(json, 'array_array_of_model') ? undefined : json['array_array_of_model'],
+        'arrayOfString': json['array_of_string'] == null ? undefined : json['array_of_string'],
+        'arrayArrayOfInteger': json['array_array_of_integer'] == null ? undefined : json['array_array_of_integer'],
+        'arrayArrayOfModel': json['array_array_of_model'] == null ? undefined : json['array_array_of_model'],
     };
 }
 
 export function ArrayTestToJSON(value?: ArrayTest | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Capitalization.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Capitalization.ts
index 30a6a48dfde4..1d7e59d6099d 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Capitalization.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Capitalization.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -70,26 +70,23 @@ export function CapitalizationFromJSON(json: any): Capitalization {
 }
 
 export function CapitalizationFromJSONTyped(json: any, ignoreDiscriminator: boolean): Capitalization {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'smallCamel': !exists(json, 'smallCamel') ? undefined : json['smallCamel'],
-        'capitalCamel': !exists(json, 'CapitalCamel') ? undefined : json['CapitalCamel'],
-        'smallSnake': !exists(json, 'small_Snake') ? undefined : json['small_Snake'],
-        'capitalSnake': !exists(json, 'Capital_Snake') ? undefined : json['Capital_Snake'],
-        'sCAETHFlowPoints': !exists(json, 'SCA_ETH_Flow_Points') ? undefined : json['SCA_ETH_Flow_Points'],
-        'aTTNAME': !exists(json, 'ATT_NAME') ? undefined : json['ATT_NAME'],
+        'smallCamel': json['smallCamel'] == null ? undefined : json['smallCamel'],
+        'capitalCamel': json['CapitalCamel'] == null ? undefined : json['CapitalCamel'],
+        'smallSnake': json['small_Snake'] == null ? undefined : json['small_Snake'],
+        'capitalSnake': json['Capital_Snake'] == null ? undefined : json['Capital_Snake'],
+        'sCAETHFlowPoints': json['SCA_ETH_Flow_Points'] == null ? undefined : json['SCA_ETH_Flow_Points'],
+        'aTTNAME': json['ATT_NAME'] == null ? undefined : json['ATT_NAME'],
     };
 }
 
 export function CapitalizationToJSON(value?: Capitalization | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Cat.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Cat.ts
index 94507ba96b1f..3bc2bd8cdb13 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Cat.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Cat.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Animal } from './Animal';
 import {
     AnimalFromJSON,
@@ -46,21 +46,18 @@ export function CatFromJSON(json: any): Cat {
 }
 
 export function CatFromJSONTyped(json: any, ignoreDiscriminator: boolean): Cat {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         ...AnimalFromJSONTyped(json, ignoreDiscriminator),
-        'declawed': !exists(json, 'declawed') ? undefined : json['declawed'],
+        'declawed': json['declawed'] == null ? undefined : json['declawed'],
     };
 }
 
 export function CatToJSON(value?: Cat | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         ...AnimalToJSON(value),
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Category.ts
index 3b4d927fb025..e571880c8e1e 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Category.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -46,22 +46,19 @@ export function CategoryFromJSON(json: any): Category {
 }
 
 export function CategoryFromJSONTyped(json: any, ignoreDiscriminator: boolean): Category {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
+        'id': json['id'] == null ? undefined : json['id'],
         'name': json['name'],
     };
 }
 
 export function CategoryToJSON(value?: Category | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ClassModel.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ClassModel.ts
index 6309f0dfce81..6d9ad88c1c55 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ClassModel.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ClassModel.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Model for testing model with "_class" property
  * @export
@@ -39,21 +39,18 @@ export function ClassModelFromJSON(json: any): ClassModel {
 }
 
 export function ClassModelFromJSONTyped(json: any, ignoreDiscriminator: boolean): ClassModel {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        '_class': !exists(json, '_class') ? undefined : json['_class'],
+        '_class': json['_class'] == null ? undefined : json['_class'],
     };
 }
 
 export function ClassModelToJSON(value?: ClassModel | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Client.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Client.ts
index 96e2993ca963..807cd0777308 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Client.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Client.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function ClientFromJSON(json: any): Client {
 }
 
 export function ClientFromJSONTyped(json: any, ignoreDiscriminator: boolean): Client {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'client': !exists(json, 'client') ? undefined : json['client'],
+        'client': json['client'] == null ? undefined : json['client'],
     };
 }
 
 export function ClientToJSON(value?: Client | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/DeprecatedObject.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/DeprecatedObject.ts
index c8113b4c9a39..8eab37673066 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/DeprecatedObject.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/DeprecatedObject.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function DeprecatedObjectFromJSON(json: any): DeprecatedObject {
 }
 
 export function DeprecatedObjectFromJSONTyped(json: any, ignoreDiscriminator: boolean): DeprecatedObject {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function DeprecatedObjectToJSON(value?: DeprecatedObject | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Dog.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Dog.ts
index fec18e3e484c..921e17cb803f 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Dog.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Dog.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Animal } from './Animal';
 import {
     AnimalFromJSON,
@@ -46,21 +46,18 @@ export function DogFromJSON(json: any): Dog {
 }
 
 export function DogFromJSONTyped(json: any, ignoreDiscriminator: boolean): Dog {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         ...AnimalFromJSONTyped(json, ignoreDiscriminator),
-        'breed': !exists(json, 'breed') ? undefined : json['breed'],
+        'breed': json['breed'] == null ? undefined : json['breed'],
     };
 }
 
 export function DogToJSON(value?: Dog | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         ...AnimalToJSON(value),
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumArrays.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumArrays.ts
index 0c93a179655a..aa7200c92542 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumArrays.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumArrays.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -65,22 +65,19 @@ export function EnumArraysFromJSON(json: any): EnumArrays {
 }
 
 export function EnumArraysFromJSONTyped(json: any, ignoreDiscriminator: boolean): EnumArrays {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'justSymbol': !exists(json, 'just_symbol') ? undefined : json['just_symbol'],
-        'arrayEnum': !exists(json, 'array_enum') ? undefined : json['array_enum'],
+        'justSymbol': json['just_symbol'] == null ? undefined : json['just_symbol'],
+        'arrayEnum': json['array_enum'] == null ? undefined : json['array_enum'],
     };
 }
 
 export function EnumArraysToJSON(value?: EnumArrays | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumTest.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumTest.ts
index 9a8b5051c321..dbe67a7cf1f7 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/EnumTest.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { OuterEnum } from './OuterEnum';
 import {
     OuterEnumFromJSON,
@@ -147,28 +147,25 @@ export function EnumTestFromJSON(json: any): EnumTest {
 }
 
 export function EnumTestFromJSONTyped(json: any, ignoreDiscriminator: boolean): EnumTest {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'enumString': !exists(json, 'enum_string') ? undefined : json['enum_string'],
+        'enumString': json['enum_string'] == null ? undefined : json['enum_string'],
         'enumStringRequired': json['enum_string_required'],
-        'enumInteger': !exists(json, 'enum_integer') ? undefined : json['enum_integer'],
-        'enumNumber': !exists(json, 'enum_number') ? undefined : json['enum_number'],
-        'outerEnum': !exists(json, 'outerEnum') ? undefined : OuterEnumFromJSON(json['outerEnum']),
-        'outerEnumInteger': !exists(json, 'outerEnumInteger') ? undefined : OuterEnumIntegerFromJSON(json['outerEnumInteger']),
-        'outerEnumDefaultValue': !exists(json, 'outerEnumDefaultValue') ? undefined : OuterEnumDefaultValueFromJSON(json['outerEnumDefaultValue']),
-        'outerEnumIntegerDefaultValue': !exists(json, 'outerEnumIntegerDefaultValue') ? undefined : OuterEnumIntegerDefaultValueFromJSON(json['outerEnumIntegerDefaultValue']),
+        'enumInteger': json['enum_integer'] == null ? undefined : json['enum_integer'],
+        'enumNumber': json['enum_number'] == null ? undefined : json['enum_number'],
+        'outerEnum': json['outerEnum'] == null ? undefined : OuterEnumFromJSON(json['outerEnum']),
+        'outerEnumInteger': json['outerEnumInteger'] == null ? undefined : OuterEnumIntegerFromJSON(json['outerEnumInteger']),
+        'outerEnumDefaultValue': json['outerEnumDefaultValue'] == null ? undefined : OuterEnumDefaultValueFromJSON(json['outerEnumDefaultValue']),
+        'outerEnumIntegerDefaultValue': json['outerEnumIntegerDefaultValue'] == null ? undefined : OuterEnumIntegerDefaultValueFromJSON(json['outerEnumIntegerDefaultValue']),
     };
 }
 
 export function EnumTestToJSON(value?: EnumTest | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FakeBigDecimalMap200Response.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FakeBigDecimalMap200Response.ts
index bf4371a93c1e..732ed05b62a0 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FakeBigDecimalMap200Response.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FakeBigDecimalMap200Response.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -45,22 +45,19 @@ export function FakeBigDecimalMap200ResponseFromJSON(json: any): FakeBigDecimalM
 }
 
 export function FakeBigDecimalMap200ResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): FakeBigDecimalMap200Response {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'someId': !exists(json, 'someId') ? undefined : json['someId'],
-        'someMap': !exists(json, 'someMap') ? undefined : json['someMap'],
+        'someId': json['someId'] == null ? undefined : json['someId'],
+        'someMap': json['someMap'] == null ? undefined : json['someMap'],
     };
 }
 
 export function FakeBigDecimalMap200ResponseToJSON(value?: FakeBigDecimalMap200Response | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FileSchemaTestClass.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FileSchemaTestClass.ts
index 10e7e77f8f87..25bf3ba4dbf1 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FileSchemaTestClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FileSchemaTestClass.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -45,22 +45,19 @@ export function FileSchemaTestClassFromJSON(json: any): FileSchemaTestClass {
 }
 
 export function FileSchemaTestClassFromJSONTyped(json: any, ignoreDiscriminator: boolean): FileSchemaTestClass {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'file': !exists(json, 'file') ? undefined : json['file'],
-        'files': !exists(json, 'files') ? undefined : json['files'],
+        'file': json['file'] == null ? undefined : json['file'],
+        'files': json['files'] == null ? undefined : json['files'],
     };
 }
 
 export function FileSchemaTestClassToJSON(value?: FileSchemaTestClass | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Foo.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Foo.ts
index ec179ce581b5..feb5e47da049 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Foo.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Foo.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function FooFromJSON(json: any): Foo {
 }
 
 export function FooFromJSONTyped(json: any, ignoreDiscriminator: boolean): Foo {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'bar': !exists(json, 'bar') ? undefined : json['bar'],
+        'bar': json['bar'] == null ? undefined : json['bar'],
     };
 }
 
 export function FooToJSON(value?: Foo | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FooGetDefaultResponse.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FooGetDefaultResponse.ts
index ffd3e185c760..b834ab550db6 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FooGetDefaultResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FooGetDefaultResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Foo } from './Foo';
 import {
     FooFromJSON,
@@ -46,21 +46,18 @@ export function FooGetDefaultResponseFromJSON(json: any): FooGetDefaultResponse
 }
 
 export function FooGetDefaultResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): FooGetDefaultResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'string': !exists(json, 'string') ? undefined : FooFromJSON(json['string']),
+        'string': json['string'] == null ? undefined : FooFromJSON(json['string']),
     };
 }
 
 export function FooGetDefaultResponseToJSON(value?: FooGetDefaultResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FormatTest.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FormatTest.ts
index b509f3575a2f..f2113709741f 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FormatTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/FormatTest.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Decimal } from './Decimal';
 import {
     DecimalFromJSON,
@@ -140,36 +140,33 @@ export function FormatTestFromJSON(json: any): FormatTest {
 }
 
 export function FormatTestFromJSONTyped(json: any, ignoreDiscriminator: boolean): FormatTest {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'integer': !exists(json, 'integer') ? undefined : json['integer'],
-        'int32': !exists(json, 'int32') ? undefined : json['int32'],
-        'int64': !exists(json, 'int64') ? undefined : json['int64'],
+        'integer': json['integer'] == null ? undefined : json['integer'],
+        'int32': json['int32'] == null ? undefined : json['int32'],
+        'int64': json['int64'] == null ? undefined : json['int64'],
         'number': json['number'],
-        '_float': !exists(json, 'float') ? undefined : json['float'],
-        '_double': !exists(json, 'double') ? undefined : json['double'],
-        'decimal': !exists(json, 'decimal') ? undefined : DecimalFromJSON(json['decimal']),
-        'string': !exists(json, 'string') ? undefined : json['string'],
+        '_float': json['float'] == null ? undefined : json['float'],
+        '_double': json['double'] == null ? undefined : json['double'],
+        'decimal': json['decimal'] == null ? undefined : DecimalFromJSON(json['decimal']),
+        'string': json['string'] == null ? undefined : json['string'],
         '_byte': json['byte'],
-        'binary': !exists(json, 'binary') ? undefined : json['binary'],
+        'binary': json['binary'] == null ? undefined : json['binary'],
         'date': (new Date(json['date'])),
-        'dateTime': !exists(json, 'dateTime') ? undefined : (new Date(json['dateTime'])),
-        'uuid': !exists(json, 'uuid') ? undefined : json['uuid'],
+        'dateTime': json['dateTime'] == null ? undefined : (new Date(json['dateTime'])),
+        'uuid': json['uuid'] == null ? undefined : json['uuid'],
         'password': json['password'],
-        'patternWithDigits': !exists(json, 'pattern_with_digits') ? undefined : json['pattern_with_digits'],
-        'patternWithDigitsAndDelimiter': !exists(json, 'pattern_with_digits_and_delimiter') ? undefined : json['pattern_with_digits_and_delimiter'],
+        'patternWithDigits': json['pattern_with_digits'] == null ? undefined : json['pattern_with_digits'],
+        'patternWithDigitsAndDelimiter': json['pattern_with_digits_and_delimiter'] == null ? undefined : json['pattern_with_digits_and_delimiter'],
     };
 }
 
 export function FormatTestToJSON(value?: FormatTest | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
@@ -184,7 +181,7 @@ export function FormatTestToJSON(value?: FormatTest | null): any {
         'byte': value['_byte'],
         'binary': value['binary'],
         'date': ((value['date']).toISOString().substring(0,10)),
-        'dateTime': !exists(value, 'dateTime') ? undefined : ((value['dateTime']).toISOString()),
+        'dateTime': value['dateTime'] == null ? undefined : ((value['dateTime']).toISOString()),
         'uuid': value['uuid'],
         'password': value['password'],
         'pattern_with_digits': value['patternWithDigits'],
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HasOnlyReadOnly.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HasOnlyReadOnly.ts
index e59842cd3e2f..9cc2da8afdaa 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HasOnlyReadOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HasOnlyReadOnly.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -45,22 +45,19 @@ export function HasOnlyReadOnlyFromJSON(json: any): HasOnlyReadOnly {
 }
 
 export function HasOnlyReadOnlyFromJSONTyped(json: any, ignoreDiscriminator: boolean): HasOnlyReadOnly {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'bar': !exists(json, 'bar') ? undefined : json['bar'],
-        'foo': !exists(json, 'foo') ? undefined : json['foo'],
+        'bar': json['bar'] == null ? undefined : json['bar'],
+        'foo': json['foo'] == null ? undefined : json['foo'],
     };
 }
 
 export function HasOnlyReadOnlyToJSON(value?: HasOnlyReadOnly | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HealthCheckResult.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HealthCheckResult.ts
index 6d58f178cdcb..83a0cf52ee15 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HealthCheckResult.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/HealthCheckResult.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Just a string to inform instance is up and running. Make it nullable in hope to get it as pointer in generated model.
  * @export
@@ -39,21 +39,18 @@ export function HealthCheckResultFromJSON(json: any): HealthCheckResult {
 }
 
 export function HealthCheckResultFromJSONTyped(json: any, ignoreDiscriminator: boolean): HealthCheckResult {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'nullableMessage': !exists(json, 'NullableMessage') ? undefined : json['NullableMessage'],
+        'nullableMessage': json['NullableMessage'] == null ? undefined : json['NullableMessage'],
     };
 }
 
 export function HealthCheckResultToJSON(value?: HealthCheckResult | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/List.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/List.ts
index c4afb61a4de4..5513deda0790 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/List.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/List.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function ListFromJSON(json: any): List {
 }
 
 export function ListFromJSONTyped(json: any, ignoreDiscriminator: boolean): List {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        '_123list': !exists(json, '123-list') ? undefined : json['123-list'],
+        '_123list': json['123-list'] == null ? undefined : json['123-list'],
     };
 }
 
 export function ListToJSON(value?: List | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MapTest.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MapTest.ts
index b36dfda98d88..0ba03f9a901b 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MapTest.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MapTest.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -68,24 +68,21 @@ export function MapTestFromJSON(json: any): MapTest {
 }
 
 export function MapTestFromJSONTyped(json: any, ignoreDiscriminator: boolean): MapTest {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'mapMapOfString': !exists(json, 'map_map_of_string') ? undefined : json['map_map_of_string'],
-        'mapOfEnumString': !exists(json, 'map_of_enum_string') ? undefined : json['map_of_enum_string'],
-        'directMap': !exists(json, 'direct_map') ? undefined : json['direct_map'],
-        'indirectMap': !exists(json, 'indirect_map') ? undefined : json['indirect_map'],
+        'mapMapOfString': json['map_map_of_string'] == null ? undefined : json['map_map_of_string'],
+        'mapOfEnumString': json['map_of_enum_string'] == null ? undefined : json['map_of_enum_string'],
+        'directMap': json['direct_map'] == null ? undefined : json['direct_map'],
+        'indirectMap': json['indirect_map'] == null ? undefined : json['indirect_map'],
     };
 }
 
 export function MapTestToJSON(value?: MapTest | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MixedPropertiesAndAdditionalPropertiesClass.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MixedPropertiesAndAdditionalPropertiesClass.ts
index f22b2b72a221..156ccc8a342d 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MixedPropertiesAndAdditionalPropertiesClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MixedPropertiesAndAdditionalPropertiesClass.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Animal } from './Animal';
 import {
     AnimalFromJSON,
@@ -58,29 +58,26 @@ export function MixedPropertiesAndAdditionalPropertiesClassFromJSON(json: any):
 }
 
 export function MixedPropertiesAndAdditionalPropertiesClassFromJSONTyped(json: any, ignoreDiscriminator: boolean): MixedPropertiesAndAdditionalPropertiesClass {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'uuid': !exists(json, 'uuid') ? undefined : json['uuid'],
-        'dateTime': !exists(json, 'dateTime') ? undefined : (new Date(json['dateTime'])),
-        'map': !exists(json, 'map') ? undefined : (mapValues(json['map'], AnimalFromJSON)),
+        'uuid': json['uuid'] == null ? undefined : json['uuid'],
+        'dateTime': json['dateTime'] == null ? undefined : (new Date(json['dateTime'])),
+        'map': json['map'] == null ? undefined : (mapValues(json['map'], AnimalFromJSON)),
     };
 }
 
 export function MixedPropertiesAndAdditionalPropertiesClassToJSON(value?: MixedPropertiesAndAdditionalPropertiesClass | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
         'uuid': value['uuid'],
-        'dateTime': !exists(value, 'dateTime') ? undefined : ((value['dateTime']).toISOString()),
-        'map': !exists(value, 'map') ? undefined : (mapValues(value['map'], AnimalToJSON)),
+        'dateTime': value['dateTime'] == null ? undefined : ((value['dateTime']).toISOString()),
+        'map': value['map'] == null ? undefined : (mapValues(value['map'], AnimalToJSON)),
     };
 }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Model200Response.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Model200Response.ts
index 8cabcd2d4b3e..22570495bd69 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Model200Response.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Model200Response.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Model for testing model name starting with number
  * @export
@@ -45,22 +45,19 @@ export function Model200ResponseFromJSON(json: any): Model200Response {
 }
 
 export function Model200ResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): Model200Response {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'name': !exists(json, 'name') ? undefined : json['name'],
-        '_class': !exists(json, 'class') ? undefined : json['class'],
+        'name': json['name'] == null ? undefined : json['name'],
+        '_class': json['class'] == null ? undefined : json['class'],
     };
 }
 
 export function Model200ResponseToJSON(value?: Model200Response | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelApiResponse.ts
index b3685aea30ad..7116a59f8d22 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelApiResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -51,23 +51,20 @@ export function ModelApiResponseFromJSON(json: any): ModelApiResponse {
 }
 
 export function ModelApiResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): ModelApiResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'code': !exists(json, 'code') ? undefined : json['code'],
-        'type': !exists(json, 'type') ? undefined : json['type'],
-        'message': !exists(json, 'message') ? undefined : json['message'],
+        'code': json['code'] == null ? undefined : json['code'],
+        'type': json['type'] == null ? undefined : json['type'],
+        'message': json['message'] == null ? undefined : json['message'],
     };
 }
 
 export function ModelApiResponseToJSON(value?: ModelApiResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelFile.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelFile.ts
index 0611bad737d6..dc08a2f1474a 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelFile.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ModelFile.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Must be named `File` for test.
  * @export
@@ -39,21 +39,18 @@ export function ModelFileFromJSON(json: any): ModelFile {
 }
 
 export function ModelFileFromJSONTyped(json: any, ignoreDiscriminator: boolean): ModelFile {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'sourceURI': !exists(json, 'sourceURI') ? undefined : json['sourceURI'],
+        'sourceURI': json['sourceURI'] == null ? undefined : json['sourceURI'],
     };
 }
 
 export function ModelFileToJSON(value?: ModelFile | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Name.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Name.ts
index 4ba736c85a45..53ca17e6317f 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Name.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Name.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Model for testing model name same as property name
  * @export
@@ -58,24 +58,21 @@ export function NameFromJSON(json: any): Name {
 }
 
 export function NameFromJSONTyped(json: any, ignoreDiscriminator: boolean): Name {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
         'name': json['name'],
-        'snakeCase': !exists(json, 'snake_case') ? undefined : json['snake_case'],
-        'property': !exists(json, 'property') ? undefined : json['property'],
-        '_123number': !exists(json, '123Number') ? undefined : json['123Number'],
+        'snakeCase': json['snake_case'] == null ? undefined : json['snake_case'],
+        'property': json['property'] == null ? undefined : json['property'],
+        '_123number': json['123Number'] == null ? undefined : json['123Number'],
     };
 }
 
 export function NameToJSON(value?: Name | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NullableClass.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NullableClass.ts
index de25ab3382a4..b64cd7d8f452 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NullableClass.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NullableClass.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -106,33 +106,30 @@ export function NullableClassFromJSON(json: any): NullableClass {
 }
 
 export function NullableClassFromJSONTyped(json: any, ignoreDiscriminator: boolean): NullableClass {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
             ...json,
-        'integerProp': !exists(json, 'integer_prop') ? undefined : json['integer_prop'],
-        'numberProp': !exists(json, 'number_prop') ? undefined : json['number_prop'],
-        'booleanProp': !exists(json, 'boolean_prop') ? undefined : json['boolean_prop'],
-        'stringProp': !exists(json, 'string_prop') ? undefined : json['string_prop'],
-        'dateProp': !exists(json, 'date_prop') ? undefined : (new Date(json['date_prop'])),
-        'datetimeProp': !exists(json, 'datetime_prop') ? undefined : (new Date(json['datetime_prop'])),
-        'arrayNullableProp': !exists(json, 'array_nullable_prop') ? undefined : json['array_nullable_prop'],
-        'arrayAndItemsNullableProp': !exists(json, 'array_and_items_nullable_prop') ? undefined : json['array_and_items_nullable_prop'],
-        'arrayItemsNullable': !exists(json, 'array_items_nullable') ? undefined : json['array_items_nullable'],
-        'objectNullableProp': !exists(json, 'object_nullable_prop') ? undefined : json['object_nullable_prop'],
-        'objectAndItemsNullableProp': !exists(json, 'object_and_items_nullable_prop') ? undefined : json['object_and_items_nullable_prop'],
-        'objectItemsNullable': !exists(json, 'object_items_nullable') ? undefined : json['object_items_nullable'],
+        'integerProp': json['integer_prop'] == null ? undefined : json['integer_prop'],
+        'numberProp': json['number_prop'] == null ? undefined : json['number_prop'],
+        'booleanProp': json['boolean_prop'] == null ? undefined : json['boolean_prop'],
+        'stringProp': json['string_prop'] == null ? undefined : json['string_prop'],
+        'dateProp': json['date_prop'] == null ? undefined : (new Date(json['date_prop'])),
+        'datetimeProp': json['datetime_prop'] == null ? undefined : (new Date(json['datetime_prop'])),
+        'arrayNullableProp': json['array_nullable_prop'] == null ? undefined : json['array_nullable_prop'],
+        'arrayAndItemsNullableProp': json['array_and_items_nullable_prop'] == null ? undefined : json['array_and_items_nullable_prop'],
+        'arrayItemsNullable': json['array_items_nullable'] == null ? undefined : json['array_items_nullable'],
+        'objectNullableProp': json['object_nullable_prop'] == null ? undefined : json['object_nullable_prop'],
+        'objectAndItemsNullableProp': json['object_and_items_nullable_prop'] == null ? undefined : json['object_and_items_nullable_prop'],
+        'objectItemsNullable': json['object_items_nullable'] == null ? undefined : json['object_items_nullable'],
     };
 }
 
 export function NullableClassToJSON(value?: NullableClass | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
@@ -141,8 +138,8 @@ export function NullableClassToJSON(value?: NullableClass | null): any {
         'number_prop': value['numberProp'],
         'boolean_prop': value['booleanProp'],
         'string_prop': value['stringProp'],
-        'date_prop': !exists(value, 'dateProp') ? undefined : ((value['dateProp'] as any).toISOString().substring(0,10)),
-        'datetime_prop': !exists(value, 'datetimeProp') ? undefined : ((value['datetimeProp'] as any).toISOString()),
+        'date_prop': value['dateProp'] == null ? undefined : ((value['dateProp'] as any).toISOString().substring(0,10)),
+        'datetime_prop': value['datetimeProp'] == null ? undefined : ((value['datetimeProp'] as any).toISOString()),
         'array_nullable_prop': value['arrayNullableProp'],
         'array_and_items_nullable_prop': value['arrayAndItemsNullableProp'],
         'array_items_nullable': value['arrayItemsNullable'],
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NumberOnly.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NumberOnly.ts
index 7a25240d3537..155afb118e43 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NumberOnly.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/NumberOnly.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function NumberOnlyFromJSON(json: any): NumberOnly {
 }
 
 export function NumberOnlyFromJSONTyped(json: any, ignoreDiscriminator: boolean): NumberOnly {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'justNumber': !exists(json, 'JustNumber') ? undefined : json['JustNumber'],
+        'justNumber': json['JustNumber'] == null ? undefined : json['JustNumber'],
     };
 }
 
 export function NumberOnlyToJSON(value?: NumberOnly | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ObjectWithDeprecatedFields.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ObjectWithDeprecatedFields.ts
index 4a921582669f..d24ea8fc197e 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ObjectWithDeprecatedFields.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ObjectWithDeprecatedFields.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { DeprecatedObject } from './DeprecatedObject';
 import {
     DeprecatedObjectFromJSON,
@@ -67,24 +67,21 @@ export function ObjectWithDeprecatedFieldsFromJSON(json: any): ObjectWithDepreca
 }
 
 export function ObjectWithDeprecatedFieldsFromJSONTyped(json: any, ignoreDiscriminator: boolean): ObjectWithDeprecatedFields {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'uuid': !exists(json, 'uuid') ? undefined : json['uuid'],
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'deprecatedRef': !exists(json, 'deprecatedRef') ? undefined : DeprecatedObjectFromJSON(json['deprecatedRef']),
-        'bars': !exists(json, 'bars') ? undefined : json['bars'],
+        'uuid': json['uuid'] == null ? undefined : json['uuid'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'deprecatedRef': json['deprecatedRef'] == null ? undefined : DeprecatedObjectFromJSON(json['deprecatedRef']),
+        'bars': json['bars'] == null ? undefined : json['bars'],
     };
 }
 
 export function ObjectWithDeprecatedFieldsToJSON(value?: ObjectWithDeprecatedFields | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Order.ts
index 6f2b79b5c85b..8921894971d6 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Order.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -81,33 +81,30 @@ export function OrderFromJSON(json: any): Order {
 }
 
 export function OrderFromJSONTyped(json: any, ignoreDiscriminator: boolean): Order {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'petId': !exists(json, 'petId') ? undefined : json['petId'],
-        'quantity': !exists(json, 'quantity') ? undefined : json['quantity'],
-        'shipDate': !exists(json, 'shipDate') ? undefined : (new Date(json['shipDate'])),
-        'status': !exists(json, 'status') ? undefined : json['status'],
-        'complete': !exists(json, 'complete') ? undefined : json['complete'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'petId': json['petId'] == null ? undefined : json['petId'],
+        'quantity': json['quantity'] == null ? undefined : json['quantity'],
+        'shipDate': json['shipDate'] == null ? undefined : (new Date(json['shipDate'])),
+        'status': json['status'] == null ? undefined : json['status'],
+        'complete': json['complete'] == null ? undefined : json['complete'],
     };
 }
 
 export function OrderToJSON(value?: Order | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': !exists(value, 'shipDate') ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
         'status': value['status'],
         'complete': value['complete'],
     };
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterComposite.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterComposite.ts
index 14b4e4542fab..49d8ad5cc5b1 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterComposite.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterComposite.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -51,23 +51,20 @@ export function OuterCompositeFromJSON(json: any): OuterComposite {
 }
 
 export function OuterCompositeFromJSONTyped(json: any, ignoreDiscriminator: boolean): OuterComposite {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'myNumber': !exists(json, 'my_number') ? undefined : json['my_number'],
-        'myString': !exists(json, 'my_string') ? undefined : json['my_string'],
-        'myBoolean': !exists(json, 'my_boolean') ? undefined : json['my_boolean'],
+        'myNumber': json['my_number'] == null ? undefined : json['my_number'],
+        'myString': json['my_string'] == null ? undefined : json['my_string'],
+        'myBoolean': json['my_boolean'] == null ? undefined : json['my_boolean'],
     };
 }
 
 export function OuterCompositeToJSON(value?: OuterComposite | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterObjectWithEnumProperty.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterObjectWithEnumProperty.ts
index 1182b747df28..3990064c3484 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterObjectWithEnumProperty.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/OuterObjectWithEnumProperty.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { OuterEnumInteger } from './OuterEnumInteger';
 import {
     OuterEnumIntegerFromJSON,
@@ -47,7 +47,7 @@ export function OuterObjectWithEnumPropertyFromJSON(json: any): OuterObjectWithE
 }
 
 export function OuterObjectWithEnumPropertyFromJSONTyped(json: any, ignoreDiscriminator: boolean): OuterObjectWithEnumProperty {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
@@ -57,11 +57,8 @@ export function OuterObjectWithEnumPropertyFromJSONTyped(json: any, ignoreDiscri
 }
 
 export function OuterObjectWithEnumPropertyToJSON(value?: OuterObjectWithEnumProperty | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Pet.ts
index e53a44fc33f5..1f29621cb3b4 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Pet.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Category } from './Category';
 import {
     CategoryFromJSON,
@@ -96,26 +96,23 @@ export function PetFromJSON(json: any): Pet {
 }
 
 export function PetFromJSONTyped(json: any, ignoreDiscriminator: boolean): Pet {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'category': !exists(json, 'category') ? undefined : CategoryFromJSON(json['category']),
+        'id': json['id'] == null ? undefined : json['id'],
+        'category': json['category'] == null ? undefined : CategoryFromJSON(json['category']),
         'name': json['name'],
         'photoUrls': json['photoUrls'],
-        'tags': !exists(json, 'tags') ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
-        'status': !exists(json, 'status') ? undefined : json['status'],
+        'tags': json['tags'] == null ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
+        'status': json['status'] == null ? undefined : json['status'],
     };
 }
 
 export function PetToJSON(value?: Pet | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
@@ -123,7 +120,7 @@ export function PetToJSON(value?: Pet | null): any {
         'category': CategoryToJSON(value['category']),
         'name': value['name'],
         'photoUrls': Array.from(value['photoUrls'] as Set<any>),
-        'tags': !exists(value, 'tags') ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
+        'tags': value['tags'] == null ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
         'status': value['status'],
     };
 }
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ReadOnlyFirst.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ReadOnlyFirst.ts
index 97c192be7826..942bcbd883ac 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ReadOnlyFirst.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/ReadOnlyFirst.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -45,22 +45,19 @@ export function ReadOnlyFirstFromJSON(json: any): ReadOnlyFirst {
 }
 
 export function ReadOnlyFirstFromJSONTyped(json: any, ignoreDiscriminator: boolean): ReadOnlyFirst {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'bar': !exists(json, 'bar') ? undefined : json['bar'],
-        'baz': !exists(json, 'baz') ? undefined : json['baz'],
+        'bar': json['bar'] == null ? undefined : json['bar'],
+        'baz': json['baz'] == null ? undefined : json['baz'],
     };
 }
 
 export function ReadOnlyFirstToJSON(value?: ReadOnlyFirst | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Return.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Return.ts
index 9d2a3787c26d..68b15542cbbd 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Return.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Return.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Model for testing reserved words
  * @export
@@ -39,21 +39,18 @@ export function ReturnFromJSON(json: any): Return {
 }
 
 export function ReturnFromJSONTyped(json: any, ignoreDiscriminator: boolean): Return {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        '_return': !exists(json, 'return') ? undefined : json['return'],
+        '_return': json['return'] == null ? undefined : json['return'],
     };
 }
 
 export function ReturnToJSON(value?: Return | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/SpecialModelName.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/SpecialModelName.ts
index 414fcb168909..b302e6f03627 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/SpecialModelName.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/SpecialModelName.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -39,21 +39,18 @@ export function SpecialModelNameFromJSON(json: any): SpecialModelName {
 }
 
 export function SpecialModelNameFromJSONTyped(json: any, ignoreDiscriminator: boolean): SpecialModelName {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        '$specialPropertyName': !exists(json, '$special[property.name]') ? undefined : json['$special[property.name]'],
+        '$specialPropertyName': json['$special[property.name]'] == null ? undefined : json['$special[property.name]'],
     };
 }
 
 export function SpecialModelNameToJSON(value?: SpecialModelName | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Tag.ts
index 446a1d0da769..4ae4af399fd8 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Tag.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -45,22 +45,19 @@ export function TagFromJSON(json: any): Tag {
 }
 
 export function TagFromJSONTyped(json: any, ignoreDiscriminator: boolean): Tag {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function TagToJSON(value?: Tag | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/User.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/User.ts
index ed2c73a101e7..c85ea3d6ed63 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/User.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -81,28 +81,25 @@ export function UserFromJSON(json: any): User {
 }
 
 export function UserFromJSONTyped(json: any, ignoreDiscriminator: boolean): User {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'username': !exists(json, 'username') ? undefined : json['username'],
-        'firstName': !exists(json, 'firstName') ? undefined : json['firstName'],
-        'lastName': !exists(json, 'lastName') ? undefined : json['lastName'],
-        'email': !exists(json, 'email') ? undefined : json['email'],
-        'password': !exists(json, 'password') ? undefined : json['password'],
-        'phone': !exists(json, 'phone') ? undefined : json['phone'],
-        'userStatus': !exists(json, 'userStatus') ? undefined : json['userStatus'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'username': json['username'] == null ? undefined : json['username'],
+        'firstName': json['firstName'] == null ? undefined : json['firstName'],
+        'lastName': json['lastName'] == null ? undefined : json['lastName'],
+        'email': json['email'] == null ? undefined : json['email'],
+        'password': json['password'] == null ? undefined : json['password'],
+        'phone': json['phone'] == null ? undefined : json['phone'],
+        'userStatus': json['userStatus'] == null ? undefined : json['userStatus'],
     };
 }
 
 export function UserToJSON(value?: User | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/runtime.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/runtime.ts
index 11b3fb68ebf9..ed35f30ef7a5 100644
--- a/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/runtime.ts
@@ -310,11 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/apis/PetApi.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/apis/PetApi.ts
index 5e23cca93b1a..c3b33a5b61df 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/apis/PetApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/apis/PetApi.ts
@@ -210,7 +210,7 @@ export class PetApi extends runtime.BaseAPI implements PetApiInterface {
      * Add a new pet to the store
      */
     async addPetRaw(requestParameters: AddPetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling addPet().'
@@ -250,7 +250,7 @@ export class PetApi extends runtime.BaseAPI implements PetApiInterface {
      * Deletes a pet
      */
     async deletePetRaw(requestParameters: DeletePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling deletePet().'
@@ -261,7 +261,7 @@ export class PetApi extends runtime.BaseAPI implements PetApiInterface {
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'apiKey')) {
+        if (requestParameters['apiKey'] != null) {
             headerParameters['api_key'] = String(requestParameters['apiKey']);
         }
 
@@ -292,7 +292,7 @@ export class PetApi extends runtime.BaseAPI implements PetApiInterface {
      * Finds Pets by status
      */
     async findPetsByStatusRaw(requestParameters: FindPetsByStatusRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] == null) {
             throw new runtime.RequiredError(
                 'status',
                 'Required parameter "status" was null or undefined when calling findPetsByStatus().'
@@ -301,7 +301,7 @@ export class PetApi extends runtime.BaseAPI implements PetApiInterface {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             queryParameters['status'] = requestParameters['status']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -337,7 +337,7 @@ export class PetApi extends runtime.BaseAPI implements PetApiInterface {
      * @deprecated
      */
     async findPetsByTagsRaw(requestParameters: FindPetsByTagsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] == null) {
             throw new runtime.RequiredError(
                 'tags',
                 'Required parameter "tags" was null or undefined when calling findPetsByTags().'
@@ -346,7 +346,7 @@ export class PetApi extends runtime.BaseAPI implements PetApiInterface {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] != null) {
             queryParameters['tags'] = requestParameters['tags']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -382,7 +382,7 @@ export class PetApi extends runtime.BaseAPI implements PetApiInterface {
      * Find pet by ID
      */
     async getPetByIdRaw(requestParameters: GetPetByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Pet>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling getPetById().'
@@ -420,7 +420,7 @@ export class PetApi extends runtime.BaseAPI implements PetApiInterface {
      * Update an existing pet
      */
     async updatePetRaw(requestParameters: UpdatePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updatePet().'
@@ -460,7 +460,7 @@ export class PetApi extends runtime.BaseAPI implements PetApiInterface {
      * Updates a pet in the store with form data
      */
     async updatePetWithFormRaw(requestParameters: UpdatePetWithFormRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling updatePetWithForm().'
@@ -490,11 +490,11 @@ export class PetApi extends runtime.BaseAPI implements PetApiInterface {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'name')) {
+        if (requestParameters['name'] != null) {
             formParams.append('name', requestParameters['name'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             formParams.append('status', requestParameters['status'] as any);
         }
 
@@ -520,7 +520,7 @@ export class PetApi extends runtime.BaseAPI implements PetApiInterface {
      * uploads an image
      */
     async uploadFileRaw(requestParameters: UploadFileRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<ModelApiResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling uploadFile().'
@@ -552,11 +552,11 @@ export class PetApi extends runtime.BaseAPI implements PetApiInterface {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'additionalMetadata')) {
+        if (requestParameters['additionalMetadata'] != null) {
             formParams.append('additionalMetadata', requestParameters['additionalMetadata'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'file')) {
+        if (requestParameters['file'] != null) {
             formParams.append('file', requestParameters['file'] as any);
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/apis/StoreApi.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/apis/StoreApi.ts
index 3b01cbdee12c..7e4e8a7b5ba8 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/apis/StoreApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/apis/StoreApi.ts
@@ -115,7 +115,7 @@ export class StoreApi extends runtime.BaseAPI implements StoreApiInterface {
      * Delete purchase order by ID
      */
     async deleteOrderRaw(requestParameters: DeleteOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling deleteOrder().'
@@ -181,7 +181,7 @@ export class StoreApi extends runtime.BaseAPI implements StoreApiInterface {
      * Find purchase order by ID
      */
     async getOrderByIdRaw(requestParameters: GetOrderByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling getOrderById().'
@@ -215,7 +215,7 @@ export class StoreApi extends runtime.BaseAPI implements StoreApiInterface {
      * Place an order for a pet
      */
     async placeOrderRaw(requestParameters: PlaceOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling placeOrder().'
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/apis/UserApi.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/apis/UserApi.ts
index fcd1170d4fa8..01ec9acf7cce 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/apis/UserApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/apis/UserApi.ts
@@ -195,7 +195,7 @@ export class UserApi extends runtime.BaseAPI implements UserApiInterface {
      * Create user
      */
     async createUserRaw(requestParameters: CreateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUser().'
@@ -231,7 +231,7 @@ export class UserApi extends runtime.BaseAPI implements UserApiInterface {
      * Creates list of users with given input array
      */
     async createUsersWithArrayInputRaw(requestParameters: CreateUsersWithArrayInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithArrayInput().'
@@ -266,7 +266,7 @@ export class UserApi extends runtime.BaseAPI implements UserApiInterface {
      * Creates list of users with given input array
      */
     async createUsersWithListInputRaw(requestParameters: CreateUsersWithListInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithListInput().'
@@ -302,7 +302,7 @@ export class UserApi extends runtime.BaseAPI implements UserApiInterface {
      * Delete user
      */
     async deleteUserRaw(requestParameters: DeleteUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling deleteUser().'
@@ -335,7 +335,7 @@ export class UserApi extends runtime.BaseAPI implements UserApiInterface {
      * Get user by user name
      */
     async getUserByNameRaw(requestParameters: GetUserByNameRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<User>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling getUserByName().'
@@ -368,14 +368,14 @@ export class UserApi extends runtime.BaseAPI implements UserApiInterface {
      * Logs user into the system
      */
     async loginUserRaw(requestParameters: LoginUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<string>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling loginUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] == null) {
             throw new runtime.RequiredError(
                 'password',
                 'Required parameter "password" was null or undefined when calling loginUser().'
@@ -384,11 +384,11 @@ export class UserApi extends runtime.BaseAPI implements UserApiInterface {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] != null) {
             queryParameters['username'] = requestParameters['username'];
         }
 
-        if (runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] != null) {
             queryParameters['password'] = requestParameters['password'];
         }
 
@@ -446,14 +446,14 @@ export class UserApi extends runtime.BaseAPI implements UserApiInterface {
      * Updated user
      */
     async updateUserRaw(requestParameters: UpdateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling updateUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updateUser().'
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Category.ts
index 001bc135bb96..19e40eaf4b86 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Category.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A category for a pet
  * @export
@@ -45,22 +45,19 @@ export function CategoryFromJSON(json: any): Category {
 }
 
 export function CategoryFromJSONTyped(json: any, ignoreDiscriminator: boolean): Category {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function CategoryToJSON(value?: Category | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/ModelApiResponse.ts
index a7fc63d81f1b..4496efe73f05 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/ModelApiResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Describes the result of uploading an image resource
  * @export
@@ -51,23 +51,20 @@ export function ModelApiResponseFromJSON(json: any): ModelApiResponse {
 }
 
 export function ModelApiResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): ModelApiResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'code': !exists(json, 'code') ? undefined : json['code'],
-        'type': !exists(json, 'type') ? undefined : json['type'],
-        'message': !exists(json, 'message') ? undefined : json['message'],
+        'code': json['code'] == null ? undefined : json['code'],
+        'type': json['type'] == null ? undefined : json['type'],
+        'message': json['message'] == null ? undefined : json['message'],
     };
 }
 
 export function ModelApiResponseToJSON(value?: ModelApiResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Order.ts
index 9e12fc242793..948cda435c8a 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Order.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * An order for a pets from the pet store
  * @export
@@ -81,33 +81,30 @@ export function OrderFromJSON(json: any): Order {
 }
 
 export function OrderFromJSONTyped(json: any, ignoreDiscriminator: boolean): Order {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'petId': !exists(json, 'petId') ? undefined : json['petId'],
-        'quantity': !exists(json, 'quantity') ? undefined : json['quantity'],
-        'shipDate': !exists(json, 'shipDate') ? undefined : (new Date(json['shipDate'])),
-        'status': !exists(json, 'status') ? undefined : json['status'],
-        'complete': !exists(json, 'complete') ? undefined : json['complete'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'petId': json['petId'] == null ? undefined : json['petId'],
+        'quantity': json['quantity'] == null ? undefined : json['quantity'],
+        'shipDate': json['shipDate'] == null ? undefined : (new Date(json['shipDate'])),
+        'status': json['status'] == null ? undefined : json['status'],
+        'complete': json['complete'] == null ? undefined : json['complete'],
     };
 }
 
 export function OrderToJSON(value?: Order | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': !exists(value, 'shipDate') ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
         'status': value['status'],
         'complete': value['complete'],
     };
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Pet.ts
index 8906602f6f4b..3c60d980598b 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Pet.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Category } from './Category';
 import {
     CategoryFromJSON,
@@ -96,26 +96,23 @@ export function PetFromJSON(json: any): Pet {
 }
 
 export function PetFromJSONTyped(json: any, ignoreDiscriminator: boolean): Pet {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'category': !exists(json, 'category') ? undefined : CategoryFromJSON(json['category']),
+        'id': json['id'] == null ? undefined : json['id'],
+        'category': json['category'] == null ? undefined : CategoryFromJSON(json['category']),
         'name': json['name'],
         'photoUrls': json['photoUrls'],
-        'tags': !exists(json, 'tags') ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
-        'status': !exists(json, 'status') ? undefined : json['status'],
+        'tags': json['tags'] == null ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
+        'status': json['status'] == null ? undefined : json['status'],
     };
 }
 
 export function PetToJSON(value?: Pet | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
@@ -123,7 +120,7 @@ export function PetToJSON(value?: Pet | null): any {
         'category': CategoryToJSON(value['category']),
         'name': value['name'],
         'photoUrls': value['photoUrls'],
-        'tags': !exists(value, 'tags') ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
+        'tags': value['tags'] == null ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
         'status': value['status'],
     };
 }
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Tag.ts
index 24a25f383dba..0a5bdbdde07c 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/Tag.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A tag for a pet
  * @export
@@ -45,22 +45,19 @@ export function TagFromJSON(json: any): Tag {
 }
 
 export function TagFromJSONTyped(json: any, ignoreDiscriminator: boolean): Tag {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function TagToJSON(value?: Tag | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/User.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/User.ts
index 00ab35b80563..3924497622f9 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/models/User.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A User who is purchasing from the pet store
  * @export
@@ -81,28 +81,25 @@ export function UserFromJSON(json: any): User {
 }
 
 export function UserFromJSONTyped(json: any, ignoreDiscriminator: boolean): User {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'username': !exists(json, 'username') ? undefined : json['username'],
-        'firstName': !exists(json, 'firstName') ? undefined : json['firstName'],
-        'lastName': !exists(json, 'lastName') ? undefined : json['lastName'],
-        'email': !exists(json, 'email') ? undefined : json['email'],
-        'password': !exists(json, 'password') ? undefined : json['password'],
-        'phone': !exists(json, 'phone') ? undefined : json['phone'],
-        'userStatus': !exists(json, 'userStatus') ? undefined : json['userStatus'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'username': json['username'] == null ? undefined : json['username'],
+        'firstName': json['firstName'] == null ? undefined : json['firstName'],
+        'lastName': json['lastName'] == null ? undefined : json['lastName'],
+        'email': json['email'] == null ? undefined : json['email'],
+        'password': json['password'] == null ? undefined : json['password'],
+        'phone': json['phone'] == null ? undefined : json['phone'],
+        'userStatus': json['userStatus'] == null ? undefined : json['userStatus'],
     };
 }
 
 export function UserToJSON(value?: User | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
index 813bf0fb4dfe..2c4103a8908e 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-interfaces/runtime.ts
@@ -310,11 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/apis/PetApi.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/apis/PetApi.ts
index 87cab8f993a2..22062ecf531f 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/apis/PetApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/apis/PetApi.ts
@@ -71,7 +71,7 @@ export class PetApi extends runtime.BaseAPI {
      * Add a new pet to the store
      */
     async addPetRaw(requestParameters: AddPetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling addPet().'
@@ -111,7 +111,7 @@ export class PetApi extends runtime.BaseAPI {
      * Deletes a pet
      */
     async deletePetRaw(requestParameters: DeletePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling deletePet().'
@@ -122,7 +122,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'apiKey')) {
+        if (requestParameters['apiKey'] != null) {
             headerParameters['api_key'] = String(requestParameters['apiKey']);
         }
 
@@ -153,7 +153,7 @@ export class PetApi extends runtime.BaseAPI {
      * Finds Pets by status
      */
     async findPetsByStatusRaw(requestParameters: FindPetsByStatusRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] == null) {
             throw new runtime.RequiredError(
                 'status',
                 'Required parameter "status" was null or undefined when calling findPetsByStatus().'
@@ -162,7 +162,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             queryParameters['status'] = requestParameters['status']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -198,7 +198,7 @@ export class PetApi extends runtime.BaseAPI {
      * @deprecated
      */
     async findPetsByTagsRaw(requestParameters: FindPetsByTagsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] == null) {
             throw new runtime.RequiredError(
                 'tags',
                 'Required parameter "tags" was null or undefined when calling findPetsByTags().'
@@ -207,7 +207,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] != null) {
             queryParameters['tags'] = requestParameters['tags']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -243,7 +243,7 @@ export class PetApi extends runtime.BaseAPI {
      * Find pet by ID
      */
     async getPetByIdRaw(requestParameters: GetPetByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Pet>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling getPetById().'
@@ -281,7 +281,7 @@ export class PetApi extends runtime.BaseAPI {
      * Update an existing pet
      */
     async updatePetRaw(requestParameters: UpdatePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updatePet().'
@@ -321,7 +321,7 @@ export class PetApi extends runtime.BaseAPI {
      * Updates a pet in the store with form data
      */
     async updatePetWithFormRaw(requestParameters: UpdatePetWithFormRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling updatePetWithForm().'
@@ -351,11 +351,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'name')) {
+        if (requestParameters['name'] != null) {
             formParams.append('name', requestParameters['name'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             formParams.append('status', requestParameters['status'] as any);
         }
 
@@ -381,7 +381,7 @@ export class PetApi extends runtime.BaseAPI {
      * uploads an image
      */
     async uploadFileRaw(requestParameters: UploadFileRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<ModelApiResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling uploadFile().'
@@ -413,11 +413,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'additionalMetadata')) {
+        if (requestParameters['additionalMetadata'] != null) {
             formParams.append('additionalMetadata', requestParameters['additionalMetadata'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'file')) {
+        if (requestParameters['file'] != null) {
             formParams.append('file', requestParameters['file'] as any);
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/apis/StoreApi.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/apis/StoreApi.ts
index 74fc267af7da..49163cc4b463 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/apis/StoreApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/apis/StoreApi.ts
@@ -44,7 +44,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Delete purchase order by ID
      */
     async deleteOrderRaw(requestParameters: DeleteOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling deleteOrder().'
@@ -110,7 +110,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Find purchase order by ID
      */
     async getOrderByIdRaw(requestParameters: GetOrderByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling getOrderById().'
@@ -144,7 +144,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Place an order for a pet
      */
     async placeOrderRaw(requestParameters: PlaceOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling placeOrder().'
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/apis/UserApi.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/apis/UserApi.ts
index 565ceefcaab9..03a5448df528 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/apis/UserApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/apis/UserApi.ts
@@ -62,7 +62,7 @@ export class UserApi extends runtime.BaseAPI {
      * Create user
      */
     async createUserRaw(requestParameters: CreateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUser().'
@@ -98,7 +98,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithArrayInputRaw(requestParameters: CreateUsersWithArrayInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithArrayInput().'
@@ -133,7 +133,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithListInputRaw(requestParameters: CreateUsersWithListInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithListInput().'
@@ -169,7 +169,7 @@ export class UserApi extends runtime.BaseAPI {
      * Delete user
      */
     async deleteUserRaw(requestParameters: DeleteUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling deleteUser().'
@@ -202,7 +202,7 @@ export class UserApi extends runtime.BaseAPI {
      * Get user by user name
      */
     async getUserByNameRaw(requestParameters: GetUserByNameRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<User>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling getUserByName().'
@@ -235,14 +235,14 @@ export class UserApi extends runtime.BaseAPI {
      * Logs user into the system
      */
     async loginUserRaw(requestParameters: LoginUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<string>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling loginUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] == null) {
             throw new runtime.RequiredError(
                 'password',
                 'Required parameter "password" was null or undefined when calling loginUser().'
@@ -251,11 +251,11 @@ export class UserApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] != null) {
             queryParameters['username'] = requestParameters['username'];
         }
 
-        if (runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] != null) {
             queryParameters['password'] = requestParameters['password'];
         }
 
@@ -313,14 +313,14 @@ export class UserApi extends runtime.BaseAPI {
      * Updated user
      */
     async updateUserRaw(requestParameters: UpdateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling updateUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updateUser().'
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Category.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Category.ts
index 001bc135bb96..19e40eaf4b86 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Category.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Category.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A category for a pet
  * @export
@@ -45,22 +45,19 @@ export function CategoryFromJSON(json: any): Category {
 }
 
 export function CategoryFromJSONTyped(json: any, ignoreDiscriminator: boolean): Category {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function CategoryToJSON(value?: Category | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/ModelApiResponse.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/ModelApiResponse.ts
index a7fc63d81f1b..4496efe73f05 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/ModelApiResponse.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/ModelApiResponse.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * Describes the result of uploading an image resource
  * @export
@@ -51,23 +51,20 @@ export function ModelApiResponseFromJSON(json: any): ModelApiResponse {
 }
 
 export function ModelApiResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): ModelApiResponse {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'code': !exists(json, 'code') ? undefined : json['code'],
-        'type': !exists(json, 'type') ? undefined : json['type'],
-        'message': !exists(json, 'message') ? undefined : json['message'],
+        'code': json['code'] == null ? undefined : json['code'],
+        'type': json['type'] == null ? undefined : json['type'],
+        'message': json['message'] == null ? undefined : json['message'],
     };
 }
 
 export function ModelApiResponseToJSON(value?: ModelApiResponse | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Order.ts
index 9e12fc242793..948cda435c8a 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Order.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * An order for a pets from the pet store
  * @export
@@ -81,33 +81,30 @@ export function OrderFromJSON(json: any): Order {
 }
 
 export function OrderFromJSONTyped(json: any, ignoreDiscriminator: boolean): Order {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'petId': !exists(json, 'petId') ? undefined : json['petId'],
-        'quantity': !exists(json, 'quantity') ? undefined : json['quantity'],
-        'shipDate': !exists(json, 'shipDate') ? undefined : (new Date(json['shipDate'])),
-        'status': !exists(json, 'status') ? undefined : json['status'],
-        'complete': !exists(json, 'complete') ? undefined : json['complete'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'petId': json['petId'] == null ? undefined : json['petId'],
+        'quantity': json['quantity'] == null ? undefined : json['quantity'],
+        'shipDate': json['shipDate'] == null ? undefined : (new Date(json['shipDate'])),
+        'status': json['status'] == null ? undefined : json['status'],
+        'complete': json['complete'] == null ? undefined : json['complete'],
     };
 }
 
 export function OrderToJSON(value?: Order | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': !exists(value, 'shipDate') ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
         'status': value['status'],
         'complete': value['complete'],
     };
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Pet.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Pet.ts
index 8906602f6f4b..3c60d980598b 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Pet.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Pet.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { Category } from './Category';
 import {
     CategoryFromJSON,
@@ -96,26 +96,23 @@ export function PetFromJSON(json: any): Pet {
 }
 
 export function PetFromJSONTyped(json: any, ignoreDiscriminator: boolean): Pet {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'category': !exists(json, 'category') ? undefined : CategoryFromJSON(json['category']),
+        'id': json['id'] == null ? undefined : json['id'],
+        'category': json['category'] == null ? undefined : CategoryFromJSON(json['category']),
         'name': json['name'],
         'photoUrls': json['photoUrls'],
-        'tags': !exists(json, 'tags') ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
-        'status': !exists(json, 'status') ? undefined : json['status'],
+        'tags': json['tags'] == null ? undefined : ((json['tags'] as Array<any>).map(TagFromJSON)),
+        'status': json['status'] == null ? undefined : json['status'],
     };
 }
 
 export function PetToJSON(value?: Pet | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
@@ -123,7 +120,7 @@ export function PetToJSON(value?: Pet | null): any {
         'category': CategoryToJSON(value['category']),
         'name': value['name'],
         'photoUrls': value['photoUrls'],
-        'tags': !exists(value, 'tags') ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
+        'tags': value['tags'] == null ? undefined : ((value['tags'] as Array<any>).map(TagToJSON)),
         'status': value['status'],
     };
 }
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Tag.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Tag.ts
index 24a25f383dba..0a5bdbdde07c 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Tag.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/Tag.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A tag for a pet
  * @export
@@ -45,22 +45,19 @@ export function TagFromJSON(json: any): Tag {
 }
 
 export function TagFromJSONTyped(json: any, ignoreDiscriminator: boolean): Tag {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'name': !exists(json, 'name') ? undefined : json['name'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'name': json['name'] == null ? undefined : json['name'],
     };
 }
 
 export function TagToJSON(value?: Tag | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/User.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/User.ts
index 00ab35b80563..3924497622f9 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/User.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/models/User.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * A User who is purchasing from the pet store
  * @export
@@ -81,28 +81,25 @@ export function UserFromJSON(json: any): User {
 }
 
 export function UserFromJSONTyped(json: any, ignoreDiscriminator: boolean): User {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'id': !exists(json, 'id') ? undefined : json['id'],
-        'username': !exists(json, 'username') ? undefined : json['username'],
-        'firstName': !exists(json, 'firstName') ? undefined : json['firstName'],
-        'lastName': !exists(json, 'lastName') ? undefined : json['lastName'],
-        'email': !exists(json, 'email') ? undefined : json['email'],
-        'password': !exists(json, 'password') ? undefined : json['password'],
-        'phone': !exists(json, 'phone') ? undefined : json['phone'],
-        'userStatus': !exists(json, 'userStatus') ? undefined : json['userStatus'],
+        'id': json['id'] == null ? undefined : json['id'],
+        'username': json['username'] == null ? undefined : json['username'],
+        'firstName': json['firstName'] == null ? undefined : json['firstName'],
+        'lastName': json['lastName'] == null ? undefined : json['lastName'],
+        'email': json['email'] == null ? undefined : json['email'],
+        'password': json['password'] == null ? undefined : json['password'],
+        'phone': json['phone'] == null ? undefined : json['phone'],
+        'userStatus': json['userStatus'] == null ? undefined : json['userStatus'],
     };
 }
 
 export function UserToJSON(value?: User | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts
index 813bf0fb4dfe..2c4103a8908e 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-npm-version/src/runtime.ts
@@ -310,11 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/with-string-enums/apis/DefaultApi.ts b/samples/client/petstore/typescript-fetch/builds/with-string-enums/apis/DefaultApi.ts
index 8cefb3e075cd..e9c2a1b83cf7 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-string-enums/apis/DefaultApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-string-enums/apis/DefaultApi.ts
@@ -63,19 +63,19 @@ export class DefaultApi extends runtime.BaseAPI {
     async fakeEnumRequestGetInlineRaw(requestParameters: FakeEnumRequestGetInlineRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<FakeEnumRequestGetInline200Response>> {
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'stringEnum')) {
+        if (requestParameters['stringEnum'] != null) {
             queryParameters['string-enum'] = requestParameters['stringEnum'];
         }
 
-        if (runtime.exists(requestParameters, 'nullableStringEnum')) {
+        if (requestParameters['nullableStringEnum'] != null) {
             queryParameters['nullable-string-enum'] = requestParameters['nullableStringEnum'];
         }
 
-        if (runtime.exists(requestParameters, 'numberEnum')) {
+        if (requestParameters['numberEnum'] != null) {
             queryParameters['number-enum'] = requestParameters['numberEnum'];
         }
 
-        if (runtime.exists(requestParameters, 'nullableNumberEnum')) {
+        if (requestParameters['nullableNumberEnum'] != null) {
             queryParameters['nullable-number-enum'] = requestParameters['nullableNumberEnum'];
         }
 
@@ -103,19 +103,19 @@ export class DefaultApi extends runtime.BaseAPI {
     async fakeEnumRequestGetRefRaw(requestParameters: FakeEnumRequestGetRefRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<EnumPatternObject>> {
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'stringEnum')) {
+        if (requestParameters['stringEnum'] != null) {
             queryParameters['string-enum'] = requestParameters['stringEnum'];
         }
 
-        if (runtime.exists(requestParameters, 'nullableStringEnum')) {
+        if (requestParameters['nullableStringEnum'] != null) {
             queryParameters['nullable-string-enum'] = requestParameters['nullableStringEnum'];
         }
 
-        if (runtime.exists(requestParameters, 'numberEnum')) {
+        if (requestParameters['numberEnum'] != null) {
             queryParameters['number-enum'] = requestParameters['numberEnum'];
         }
 
-        if (runtime.exists(requestParameters, 'nullableNumberEnum')) {
+        if (requestParameters['nullableNumberEnum'] != null) {
             queryParameters['nullable-number-enum'] = requestParameters['nullableNumberEnum'];
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/EnumPatternObject.ts b/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/EnumPatternObject.ts
index 38d190ba0f39..a902fb9c9af3 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/EnumPatternObject.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/EnumPatternObject.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 import type { NumberEnum } from './NumberEnum';
 import {
     NumberEnumFromJSON,
@@ -70,24 +70,21 @@ export function EnumPatternObjectFromJSON(json: any): EnumPatternObject {
 }
 
 export function EnumPatternObjectFromJSONTyped(json: any, ignoreDiscriminator: boolean): EnumPatternObject {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'stringEnum': !exists(json, 'string-enum') ? undefined : StringEnumFromJSON(json['string-enum']),
-        'nullableStringEnum': !exists(json, 'nullable-string-enum') ? undefined : StringEnumFromJSON(json['nullable-string-enum']),
-        'numberEnum': !exists(json, 'number-enum') ? undefined : NumberEnumFromJSON(json['number-enum']),
-        'nullableNumberEnum': !exists(json, 'nullable-number-enum') ? undefined : NumberEnumFromJSON(json['nullable-number-enum']),
+        'stringEnum': json['string-enum'] == null ? undefined : StringEnumFromJSON(json['string-enum']),
+        'nullableStringEnum': json['nullable-string-enum'] == null ? undefined : StringEnumFromJSON(json['nullable-string-enum']),
+        'numberEnum': json['number-enum'] == null ? undefined : NumberEnumFromJSON(json['number-enum']),
+        'nullableNumberEnum': json['nullable-number-enum'] == null ? undefined : NumberEnumFromJSON(json['nullable-number-enum']),
     };
 }
 
 export function EnumPatternObjectToJSON(value?: EnumPatternObject | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/FakeEnumRequestGetInline200Response.ts b/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/FakeEnumRequestGetInline200Response.ts
index cdacb3338c5c..a801fcf5ef18 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/FakeEnumRequestGetInline200Response.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-string-enums/models/FakeEnumRequestGetInline200Response.ts
@@ -12,7 +12,7 @@
  * Do not edit the class manually.
  */
 
-import { exists, mapValues } from '../runtime';
+import { mapValues } from '../runtime';
 /**
  * 
  * @export
@@ -95,24 +95,21 @@ export function FakeEnumRequestGetInline200ResponseFromJSON(json: any): FakeEnum
 }
 
 export function FakeEnumRequestGetInline200ResponseFromJSONTyped(json: any, ignoreDiscriminator: boolean): FakeEnumRequestGetInline200Response {
-    if (json === undefined || json === null) {
+    if (json == null) {
         return json;
     }
     return {
         
-        'stringEnum': !exists(json, 'string-enum') ? undefined : json['string-enum'],
-        'nullableStringEnum': !exists(json, 'nullable-string-enum') ? undefined : json['nullable-string-enum'],
-        'numberEnum': !exists(json, 'number-enum') ? undefined : json['number-enum'],
-        'nullableNumberEnum': !exists(json, 'nullable-number-enum') ? undefined : json['nullable-number-enum'],
+        'stringEnum': json['string-enum'] == null ? undefined : json['string-enum'],
+        'nullableStringEnum': json['nullable-string-enum'] == null ? undefined : json['nullable-string-enum'],
+        'numberEnum': json['number-enum'] == null ? undefined : json['number-enum'],
+        'nullableNumberEnum': json['nullable-number-enum'] == null ? undefined : json['nullable-number-enum'],
     };
 }
 
 export function FakeEnumRequestGetInline200ResponseToJSON(value?: FakeEnumRequestGetInline200Response | null): any {
-    if (value === undefined) {
-        return undefined;
-    }
-    if (value === null) {
-        return null;
+    if (value == null) {
+        return value;
     }
     return {
         
diff --git a/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts b/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts
index 4f1da922f6df..7f99552816fe 100644
--- a/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/with-string-enums/runtime.ts
@@ -310,11 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-export function exists(json: any, key: string): boolean {
-    const value = json[key];
-    return value !== undefined && value !== null;
-}
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))
diff --git a/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/apis/PetApi.ts b/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/apis/PetApi.ts
index 8bef5a9c322c..29ef7179b719 100644
--- a/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/apis/PetApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/apis/PetApi.ts
@@ -65,7 +65,7 @@ export class PetApi extends runtime.BaseAPI {
      * Add a new pet to the store
      */
     async addPetRaw(requestParameters: AddPetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling addPet().'
@@ -105,7 +105,7 @@ export class PetApi extends runtime.BaseAPI {
      * Deletes a pet
      */
     async deletePetRaw(requestParameters: DeletePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling deletePet().'
@@ -116,7 +116,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const headerParameters: runtime.HTTPHeaders = {};
 
-        if (runtime.exists(requestParameters, 'apiKey')) {
+        if (requestParameters['apiKey'] != null) {
             headerParameters['api_key'] = String(requestParameters['apiKey']);
         }
 
@@ -147,7 +147,7 @@ export class PetApi extends runtime.BaseAPI {
      * Finds Pets by status
      */
     async findPetsByStatusRaw(requestParameters: FindPetsByStatusRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] == null) {
             throw new runtime.RequiredError(
                 'status',
                 'Required parameter "status" was null or undefined when calling findPetsByStatus().'
@@ -156,7 +156,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             queryParameters['status'] = requestParameters['status']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -192,7 +192,7 @@ export class PetApi extends runtime.BaseAPI {
      * @deprecated
      */
     async findPetsByTagsRaw(requestParameters: FindPetsByTagsRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Array<Pet>>> {
-        if (!runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] == null) {
             throw new runtime.RequiredError(
                 'tags',
                 'Required parameter "tags" was null or undefined when calling findPetsByTags().'
@@ -201,7 +201,7 @@ export class PetApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'tags')) {
+        if (requestParameters['tags'] != null) {
             queryParameters['tags'] = requestParameters['tags']!.join(runtime.COLLECTION_FORMATS["csv"]);
         }
 
@@ -237,7 +237,7 @@ export class PetApi extends runtime.BaseAPI {
      * Find pet by ID
      */
     async getPetByIdRaw(requestParameters: GetPetByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Pet>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling getPetById().'
@@ -275,7 +275,7 @@ export class PetApi extends runtime.BaseAPI {
      * Update an existing pet
      */
     async updatePetRaw(requestParameters: UpdatePetRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updatePet().'
@@ -315,7 +315,7 @@ export class PetApi extends runtime.BaseAPI {
      * Updates a pet in the store with form data
      */
     async updatePetWithFormRaw(requestParameters: UpdatePetWithFormRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling updatePetWithForm().'
@@ -345,11 +345,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'name')) {
+        if (requestParameters['name'] != null) {
             formParams.append('name', requestParameters['name'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'status')) {
+        if (requestParameters['status'] != null) {
             formParams.append('status', requestParameters['status'] as any);
         }
 
@@ -375,7 +375,7 @@ export class PetApi extends runtime.BaseAPI {
      * uploads an image
      */
     async uploadFileRaw(requestParameters: UploadFileRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<ModelApiResponse>> {
-        if (!runtime.exists(requestParameters, 'petId')) {
+        if (requestParameters['petId'] == null) {
             throw new runtime.RequiredError(
                 'petId',
                 'Required parameter "petId" was null or undefined when calling uploadFile().'
@@ -407,11 +407,11 @@ export class PetApi extends runtime.BaseAPI {
             formParams = new URLSearchParams();
         }
 
-        if (runtime.exists(requestParameters, 'additionalMetadata')) {
+        if (requestParameters['additionalMetadata'] != null) {
             formParams.append('additionalMetadata', requestParameters['additionalMetadata'] as any);
         }
 
-        if (runtime.exists(requestParameters, 'file')) {
+        if (requestParameters['file'] != null) {
             formParams.append('file', requestParameters['file'] as any);
         }
 
diff --git a/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/apis/StoreApi.ts b/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/apis/StoreApi.ts
index e5443fc60605..24740b197638 100644
--- a/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/apis/StoreApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/apis/StoreApi.ts
@@ -40,7 +40,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Delete purchase order by ID
      */
     async deleteOrderRaw(requestParameters: DeleteOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling deleteOrder().'
@@ -106,7 +106,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Find purchase order by ID
      */
     async getOrderByIdRaw(requestParameters: GetOrderByIdRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'orderId')) {
+        if (requestParameters['orderId'] == null) {
             throw new runtime.RequiredError(
                 'orderId',
                 'Required parameter "orderId" was null or undefined when calling getOrderById().'
@@ -140,7 +140,7 @@ export class StoreApi extends runtime.BaseAPI {
      * Place an order for a pet
      */
     async placeOrderRaw(requestParameters: PlaceOrderRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<Order>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling placeOrder().'
diff --git a/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/apis/UserApi.ts b/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/apis/UserApi.ts
index 42175e737dee..283e3259aed0 100644
--- a/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/apis/UserApi.ts
+++ b/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/apis/UserApi.ts
@@ -58,7 +58,7 @@ export class UserApi extends runtime.BaseAPI {
      * Create user
      */
     async createUserRaw(requestParameters: CreateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUser().'
@@ -94,7 +94,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithArrayInputRaw(requestParameters: CreateUsersWithArrayInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithArrayInput().'
@@ -129,7 +129,7 @@ export class UserApi extends runtime.BaseAPI {
      * Creates list of users with given input array
      */
     async createUsersWithListInputRaw(requestParameters: CreateUsersWithListInputRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling createUsersWithListInput().'
@@ -165,7 +165,7 @@ export class UserApi extends runtime.BaseAPI {
      * Delete user
      */
     async deleteUserRaw(requestParameters: DeleteUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling deleteUser().'
@@ -198,7 +198,7 @@ export class UserApi extends runtime.BaseAPI {
      * Get user by user name
      */
     async getUserByNameRaw(requestParameters: GetUserByNameRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<User>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling getUserByName().'
@@ -231,14 +231,14 @@ export class UserApi extends runtime.BaseAPI {
      * Logs user into the system
      */
     async loginUserRaw(requestParameters: LoginUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<string>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling loginUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] == null) {
             throw new runtime.RequiredError(
                 'password',
                 'Required parameter "password" was null or undefined when calling loginUser().'
@@ -247,11 +247,11 @@ export class UserApi extends runtime.BaseAPI {
 
         const queryParameters: any = {};
 
-        if (runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] != null) {
             queryParameters['username'] = requestParameters['username'];
         }
 
-        if (runtime.exists(requestParameters, 'password')) {
+        if (requestParameters['password'] != null) {
             queryParameters['password'] = requestParameters['password'];
         }
 
@@ -309,14 +309,14 @@ export class UserApi extends runtime.BaseAPI {
      * Updated user
      */
     async updateUserRaw(requestParameters: UpdateUserRequest, initOverrides?: RequestInit | runtime.InitOverrideFunction): Promise<runtime.ApiResponse<void>> {
-        if (!runtime.exists(requestParameters, 'username')) {
+        if (requestParameters['username'] == null) {
             throw new runtime.RequiredError(
                 'username',
                 'Required parameter "username" was null or undefined when calling updateUser().'
             );
         }
 
-        if (!runtime.exists(requestParameters, 'body')) {
+        if (requestParameters['body'] == null) {
             throw new runtime.RequiredError(
                 'body',
                 'Required parameter "body" was null or undefined when calling updateUser().'
diff --git a/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/runtime.ts b/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/runtime.ts
index f18e2d18a03c..dc5ead050878 100644
--- a/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/runtime.ts
+++ b/samples/client/petstore/typescript-fetch/builds/without-runtime-checks/src/runtime.ts
@@ -310,7 +310,6 @@ export interface RequestOpts {
     body?: HTTPBody;
 }
 
-
 export function querystring(params: HTTPQuery, prefix: string = ''): string {
     return Object.keys(params)
         .map(key => querystringSingleKey(key, params[key], prefix))