Method fn

packages/jest-mock/src/index.ts:1082–1092

(implementation?: T)

  }

  fn<T extends FunctionLike = UnknownFunction>(implementation?: T): Mock<T> {
    const length = implementation ? implementation.length : 0;
    const fn = this._makeComponent<T>({
      length,
      type: 'function',
    });
    if (implementation) {
      fn.mockImplementation(implementation);
    }
    return fn;
  }

  spyOn<
    T extends object,