Method isMockFunction

packages/jest-mock/src/index.ts:1078–1080

(fn: unknown)

  ): fn is Mock<(...args: P) => R>;
  isMockFunction(fn: unknown): fn is Mock<UnknownFunction>;
  isMockFunction(fn: unknown): fn is Mock<UnknownFunction> {
    return fn != null && (fn as Mock)._isMockFunction === true;
  }

  fn<T extends FunctionLike = UnknownFunction>(implementation?: T): Mock<T> {
    const length = implementation ? implementation.length : 0;