feat: KnownKeys<T>

README.md

@@ -66,6 +66,7 @@ Provides additional types and type adjusted utilities for `typescript`
 - `Except<T, K>`: Deprecated. Same as `Omit<T, K>`.
 - `ExcludePropType<T, U>`: excludes type `U` from properties in `T`.
 - `KeyofOptional<T>`: `keyof` that works with `Record<any, any> | undefined`.
+- `KnownKeys<T>`: extract known (defined) keys from type `T`.
 - `Omit<T, K>`: From `T`, pick a set of properties whose keys are not in the union `K`. This is the opposite of `Pick<T, K>`.
 - `PartialExcept<T, U>`: Deprecated. Same as `PartialOmit<T, U>`.
 - `PartialOmit<T, U>`: makes the properties not specified in `U` becomes optional.

src/KnownKeys.spec.ts

@@ -0,0 +1,39 @@
+import { assertType, KnownKeys } from '.'
+
+test('pick out only known keys', () => {
+  type A = {
+    a?: boolean,
+    b?: number,
+    [k: string]: any,
+  }
+
+  const input: A = {}
+  const actual = getKnownKeys(input)
+  assertType<'a' | 'b'>(actual)
+})
+
+test('primitive type yields never', () => {
+  assertType.isNever(getKnownKeys(undefined))
+  assertType.isNever(getKnownKeys(true))
+  assertType.isNever(getKnownKeys(false))
+  assertType.isNever(getKnownKeys(null))
+  assertType.isNever(getKnownKeys('str'))
+  assertType.isNever(getKnownKeys(1))
+  assertType.isNever(getKnownKeys({}))
+  assertType.isNever(getKnownKeys([]))
+})
+
+test('literal gets keys', () => {
+  assertType<'a'>(getKnownKeys({ a: 1 }))
+})
+
+test('empty record yields never', () => {
+  const x: Record<any, any> = {}
+  const actual = getKnownKeys(x)
+  assertType.isNever(actual)
+})
+
+
+function getKnownKeys<T>(_value: T): KnownKeys<T> {
+  return {} as any
+}

src/KnownKeys.ts

@@ -0,0 +1,7 @@
+// by Klaus Meinhardt @ajafff
+// known from Gerrit Birkeland @Gerrit0
+// https://github.com/Microsoft/TypeScript/issues/25987#issuecomment-408339599
+// https://github.com/microsoft/TypeScript/issues/25987#issuecomment-441224690
+export type KnownKeys<T> = {
+  [K in keyof T]: string extends K ? never : number extends K ? never : K
+} extends { [_ in keyof T]: infer U } ? ({} extends U ? never : U) : never;

src/index.ts

@@ -21,6 +21,7 @@ export * from './JSONTypes';
 export * from './KeyofOptional';
 export * from './KeysWithDiffType';
 export * from './KeyTypes';
+export * from './KnownKeys';
 export * from './literalArray';
 export * from './mapKey';
 export * from './mapSeries';