[Typescript] 95. Hard - Required Keys

Implement the advanced util type RequiredKeys<T>, which picks all the required keys into a union.

For example

type Result = RequiredKeys<{ foo: number; bar?: string }>;
// expected to be “foo”

 

/* _____________ Your Code Here _____________ */

type RequiredKeys<T extends Record<PropertyKey, any>> = keyof {
  [Key in keyof T as T[Key] extends Required<T>[Key] ? Key: never]: any
}


/* _____________ Test Cases _____________ */
import type { Equal, Expect } from '@type-challenges/utils'

type cases = [
  Expect<Equal<RequiredKeys<{ a: number; b?: string }>, 'a'>>,
  Expect<Equal<RequiredKeys<{ a: undefined; b?: undefined }>, 'a'>>,
  Expect<Equal<RequiredKeys<{ a: undefined; b?: undefined; c: string; d: null }>, 'a' | 'c' | 'd'>>,
  Expect<Equal<RequiredKeys<{}>, never>>,
]