00089-hard-required-keys.ts

// ============= Test Cases =============
import type { Equal, Expect } from './test-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>>,
]


// ============= Your Code Here =============
type RequiredKeys<T> = keyof {
  [K in keyof T as T[K] extends Required<T>[K]? K : never]: T[K]
}