TS: KeysAssignableType

index.ts

// copyright 2023 Zach Hardesty

// want to check this out in the TypeScript playground?
// visit the following link to automatically see the latest version!
// https://www.typescriptlang.org/play?jsx=0#gist/63e56cd69e55a659120ec1e95a380a1e

/**
 * Extracts keys of `TObj` that correspond to values that some type in the union
 * `TTypes` can be assigned to
 *
 * @template TObj Object to extract keys from
 * @template TTypes Type to check for assignability
 * @returns Union of keys of `TObj` that correspond to values that `TTypes` can be
 *   assigned to
 */
type KeysAssignableType<TObj, TTypes> = {
  [P in keyof Required<TObj>]: TTypes extends Required<TObj>[P] ? P : never
}[keyof TObj]

type KeysAssignableExactType<TObj, TTypes> = {
  [P in keyof Required<TObj>]: [TTypes] extends [Required<TObj>[P]] ? P : never
}[keyof TObj]

interface ITestObj1 {
  a: string
  b: number
  c?: string
  d?: number
  e: boolean
  f: string | number
  g: undefined
  h: null
  i?: never
  j: unknown
  k: any
  l: { a: string }
  m: "a" | "b"
}

type T0_0 = Expect<KeysAssignableType<ITestObj1, string>, "a" | "c" | "f" | "j" | "k">
type T1_0 = Expect<
  KeysAssignableType<ITestObj1, string | number>,
  "a" | "b" | "c" | "d" | "f" | "j" | "k"
  // REVIEW: more appropriate stricter type?
  // "f" | "j" | "k"
>
type T2_0 = Expect<KeysAssignableType<ITestObj1, any>, keyof ITestObj1>
type T3_0 = Expect<KeysAssignableType<ITestObj1, object>, "j" | "k">
type T4_0 = Expect<KeysAssignableType<{}, any>, never>
type T5_0 = Expect<
  KeysAssignableType<ITestObj1, { a: string; b: number }>,
  "j" | "k" | "l"
>
type T6_0 = Expect<
  KeysAssignableType<ITestObj1, "a">,
  "a" | "c" | "f" | "j" | "k" | "m"
>
type T7_0 = Expect<
  KeysAssignableType<ITestObj1, "a" | "b" | "c">,
  "a" | "c" | "f" | "j" | "k" | "m"
  // REVIEW: more appropriate stricter type?
  // "a" | "c" | "f" | "j" | "k"
>
type T8_0 = Expect<
  KeysAssignableType<ITestObj1, "a" | number>,
  "a" | "b" | "c" | "d" | "f" | "j" | "k" | "m"
>

/** @see https://gist.github.com/zachhardesty7/1b5b6f6de328f1bb7c78cbe96ba5c720 */
type Expect<
  TInput extends TIntersect,
  TExpected extends TIntersect,
  TIntersect = TExpected & TInput,
> = TInput