Mutable Keys

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题目

Implement the advanced util type MutableKeys<T>, which picks all the mutable (not readonly) keys into a union.

For example:

type Keys = MutableKeys<{ readonly foo: string; bar: number }>;
// expected to be “bar”

解答

type MyEqual<X, Y> = (<T>() => T extends X ? 1 : 2) extends <T>() => T extends Y
  ? 1
  : 2
  ? true
  : false;

type MutableKeys<T> = keyof {
  [Key in keyof T as MyEqual<Pick<T, Key>, Readonly<Pick<T, Key>>> extends true
    ? never
    : Key]: any;
};