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 b28503d5dcb2..cef7f6bd1959 100644
--- a/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache
+++ b/modules/openapi-generator/src/main/resources/typescript-fetch/modelGeneric.mustache
@@ -141,10 +141,10 @@ export function {{classname}}ToJSONTyped(value?: {{#hasReadOnly}}Omit<{{classnam
         {{^isReadOnly}}
         {{#isPrimitiveType}}
         {{#isDateType}}
-        '{{baseName}}': {{^required}}{{#isNullable}}value['{{name}}'] === null ? null : {{/isNullable}}{{^isNullable}}value['{{name}}'] == null ? undefined : {{/isNullable}}{{/required}}((value['{{name}}']{{#isNullable}} as any{{/isNullable}}){{^required}}{{#isNullable}}?{{/isNullable}}{{/required}}.toISOString().substring(0,10)),
+        '{{baseName}}': {{^required}}value['{{name}}'] == null ? value['{{name}}'] : {{/required}}{{#isNullable}}{{#required}}value['{{name}}'] == null ? value['{{name}}'] : {{/required}}{{/isNullable}}value['{{name}}'].toISOString().substring(0,10),
         {{/isDateType}}
         {{#isDateTimeType}}
-        '{{baseName}}': {{^required}}{{#isNullable}}value['{{name}}'] === null ? null : {{/isNullable}}{{^isNullable}}value['{{name}}'] == null ? undefined : {{/isNullable}}{{/required}}((value['{{name}}']{{#isNullable}} as any{{/isNullable}}){{^required}}{{#isNullable}}?{{/isNullable}}{{/required}}.toISOString()),
+        '{{baseName}}': {{^required}}value['{{name}}'] == null ? value['{{name}}'] : {{/required}}{{#isNullable}}{{#required}}value['{{name}}'] == null ? value['{{name}}'] : {{/required}}{{/isNullable}}value['{{name}}'].toISOString(),
         {{/isDateTimeType}}
         {{#isArray}}
         '{{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}},
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 33aed92595aa..f695e974f39d 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
@@ -178,8 +178,8 @@ export function FormatTestToJSONTyped(value?: FormatTest | null, ignoreDiscrimin
         'string': value['string'],
         'byte': value['_byte'],
         'binary': value['binary'],
-        'date': ((value['date']).toISOString().substring(0,10)),
-        'dateTime': value['dateTime'] == null ? undefined : ((value['dateTime']).toISOString()),
+        'date': value['date'].toISOString().substring(0,10),
+        'dateTime': value['dateTime'] == null ? value['dateTime'] : 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/MixedPropertiesAndAdditionalPropertiesClass.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/MixedPropertiesAndAdditionalPropertiesClass.ts
index 5466bc5177e2..9263d67cc6f5 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
@@ -82,7 +82,7 @@ export function MixedPropertiesAndAdditionalPropertiesClassToJSONTyped(value?: M
     return {
         
         'uuid': value['uuid'],
-        'dateTime': value['dateTime'] == null ? undefined : ((value['dateTime']).toISOString()),
+        'dateTime': value['dateTime'] == null ? value['dateTime'] : 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/NullableClass.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/NullableClass.ts
index 425d7be4693d..11b2fb30dae8 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
@@ -143,8 +143,8 @@ export function NullableClassToJSONTyped(value?: NullableClass | null, ignoreDis
         'number_prop': value['numberProp'],
         'boolean_prop': value['booleanProp'],
         'string_prop': value['stringProp'],
-        'date_prop': value['dateProp'] === null ? null : ((value['dateProp'] as any)?.toISOString().substring(0,10)),
-        'datetime_prop': value['datetimeProp'] === null ? null : ((value['datetimeProp'] as any)?.toISOString()),
+        'date_prop': value['dateProp'] == null ? value['dateProp'] : value['dateProp'].toISOString().substring(0,10),
+        'datetime_prop': value['datetimeProp'] == null ? value['datetimeProp'] : value['datetimeProp'].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/Order.ts b/samples/client/petstore/typescript-fetch/builds/default-v3.0/models/Order.ts
index 2e9a77af0484..9444d5346722 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
@@ -109,7 +109,7 @@ export function OrderToJSONTyped(value?: Order | null, ignoreDiscriminator: bool
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? value['shipDate'] : value['shipDate'].toISOString(),
         'status': value['status'],
         'complete': value['complete'],
     };
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 46977acb9c08..42f520745eb0 100644
--- a/samples/client/petstore/typescript-fetch/builds/default/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/default/models/Order.ts
@@ -109,7 +109,7 @@ export function OrderToJSONTyped(value?: Order | null, ignoreDiscriminator: bool
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? value['shipDate'] : value['shipDate'].toISOString(),
         'status': value['status'],
         'complete': value['complete'],
     };
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 46977acb9c08..42f520745eb0 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
@@ -109,7 +109,7 @@ export function OrderToJSONTyped(value?: Order | null, ignoreDiscriminator: bool
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? value['shipDate'] : value['shipDate'].toISOString(),
         'status': value['status'],
         'complete': value['complete'],
     };
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 46977acb9c08..42f520745eb0 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
@@ -109,7 +109,7 @@ export function OrderToJSONTyped(value?: Order | null, ignoreDiscriminator: bool
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? value['shipDate'] : value['shipDate'].toISOString(),
         'status': value['status'],
         'complete': value['complete'],
     };
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 46977acb9c08..42f520745eb0 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
@@ -109,7 +109,7 @@ export function OrderToJSONTyped(value?: Order | null, ignoreDiscriminator: bool
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? value['shipDate'] : value['shipDate'].toISOString(),
         'status': value['status'],
         'complete': value['complete'],
     };
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 46977acb9c08..42f520745eb0 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
@@ -109,7 +109,7 @@ export function OrderToJSONTyped(value?: Order | null, ignoreDiscriminator: bool
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? value['shipDate'] : value['shipDate'].toISOString(),
         'status': value['status'],
         'complete': value['complete'],
     };
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 33aed92595aa..f695e974f39d 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
@@ -178,8 +178,8 @@ export function FormatTestToJSONTyped(value?: FormatTest | null, ignoreDiscrimin
         'string': value['string'],
         'byte': value['_byte'],
         'binary': value['binary'],
-        'date': ((value['date']).toISOString().substring(0,10)),
-        'dateTime': value['dateTime'] == null ? undefined : ((value['dateTime']).toISOString()),
+        'date': value['date'].toISOString().substring(0,10),
+        'dateTime': value['dateTime'] == null ? value['dateTime'] : 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/MixedPropertiesAndAdditionalPropertiesClass.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/MixedPropertiesAndAdditionalPropertiesClass.ts
index 5466bc5177e2..9263d67cc6f5 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
@@ -82,7 +82,7 @@ export function MixedPropertiesAndAdditionalPropertiesClassToJSONTyped(value?: M
     return {
         
         'uuid': value['uuid'],
-        'dateTime': value['dateTime'] == null ? undefined : ((value['dateTime']).toISOString()),
+        'dateTime': value['dateTime'] == null ? value['dateTime'] : value['dateTime'].toISOString(),
         'map': value['map'] == null ? undefined : (mapValues(value['map'], AnimalToJSON)),
     };
 }
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 425d7be4693d..11b2fb30dae8 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
@@ -143,8 +143,8 @@ export function NullableClassToJSONTyped(value?: NullableClass | null, ignoreDis
         'number_prop': value['numberProp'],
         'boolean_prop': value['booleanProp'],
         'string_prop': value['stringProp'],
-        'date_prop': value['dateProp'] === null ? null : ((value['dateProp'] as any)?.toISOString().substring(0,10)),
-        'datetime_prop': value['datetimeProp'] === null ? null : ((value['datetimeProp'] as any)?.toISOString()),
+        'date_prop': value['dateProp'] == null ? value['dateProp'] : value['dateProp'].toISOString().substring(0,10),
+        'datetime_prop': value['datetimeProp'] == null ? value['datetimeProp'] : value['datetimeProp'].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/Order.ts b/samples/client/petstore/typescript-fetch/builds/snakecase-discriminator/models/Order.ts
index 2e9a77af0484..9444d5346722 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
@@ -109,7 +109,7 @@ export function OrderToJSONTyped(value?: Order | null, ignoreDiscriminator: bool
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? value['shipDate'] : value['shipDate'].toISOString(),
         'status': value['status'],
         'complete': value['complete'],
     };
diff --git a/samples/client/petstore/typescript-fetch/builds/validation-attributes/models/Order.ts b/samples/client/petstore/typescript-fetch/builds/validation-attributes/models/Order.ts
index 072d3bf5ac9e..097b74d85c79 100644
--- a/samples/client/petstore/typescript-fetch/builds/validation-attributes/models/Order.ts
+++ b/samples/client/petstore/typescript-fetch/builds/validation-attributes/models/Order.ts
@@ -112,7 +112,7 @@ export function OrderToJSONTyped(value?: Order | null, ignoreDiscriminator: bool
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? value['shipDate'] : value['shipDate'].toISOString(),
         'status': value['status'],
         'complete': value['complete'],
     };
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 46977acb9c08..42f520745eb0 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
@@ -109,7 +109,7 @@ export function OrderToJSONTyped(value?: Order | null, ignoreDiscriminator: bool
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? value['shipDate'] : value['shipDate'].toISOString(),
         'status': value['status'],
         'complete': value['complete'],
     };
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 46977acb9c08..42f520745eb0 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
@@ -109,7 +109,7 @@ export function OrderToJSONTyped(value?: Order | null, ignoreDiscriminator: bool
         'id': value['id'],
         'petId': value['petId'],
         'quantity': value['quantity'],
-        'shipDate': value['shipDate'] == null ? undefined : ((value['shipDate']).toISOString()),
+        'shipDate': value['shipDate'] == null ? value['shipDate'] : value['shipDate'].toISOString(),
         'status': value['status'],
         'complete': value['complete'],
     };