IsRequiredKey

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题目

Implement a generic IsRequiredKey<T, K> that return whether K are required keys of T .

For example

type A = IsRequiredKey<{ a: number; b?: string }, "a">; // true
type B = IsRequiredKey<{ a: number; b?: string }, "b">; // false
type C = IsRequiredKey<{ a: number; b?: string }, "b" | "a">; // false

解答

type IsRequiredKey<T, K extends keyof T> = T extends Record<K, T[K]>
  ? true
  : false;