jest mock implementation

1describe("mockImplementation", () => {
2  test("function", () => {
3    const mockFn1 = jest.fn().mockImplementation(() => 42);
4    const mockFn2 = jest.fn(() => 42);
5
6    expect(mockFn1()).toBe(42);
7    expect(mockFn2()).toBe(42);
8  });

1import randomColor from "randomcolor";
2
3
4jest.mock("randomColor", () => {
5  return {
6    randomColor: () => {
7      return mockColor('#123456');
8    }
9  }
10});
11let mockColor = jest.fn();

1test("es6 class", () => {
2    const SomeClass = jest.fn();
3    const mMock = jest.fn();
4
5    SomeClass.mockImplementation(() => {
6      return {
7        m: mMock
8      };
9    });
10
11    const some = new SomeClass();
12    some.m("a", "b");
13    expect(mMock.mock.calls).toEqual([["a", "b"]]);
14  });

1import Foo from './Foo';
2import Bar from './Bar';
3
4jest.mock('./Bar');
5
6describe('Foo', () => {
7  it('should return correct foo', () => {
8    // As Bar is already mocked,
9    // we just need to cast it to jest.Mock (for TypeScript) and mock whatever you want
10    (Bar.prototype.runBar as jest.Mock).mockReturnValue('Mocked bar');
11    const foo = new Foo();
12    expect(foo.runFoo()).toBe('real foo : Mocked bar');
13  });
14});
15
16
17

1test("mock.calls", () => {
2  const mockFn = jest.fn();
3  mockFn(1, 2);
4
5  expect(mockFn.mock.calls).toEqual([[1, 2]]);
6});

1test("mock.instances", () => {
2  const mockFn = jest.fn();
3
4  const a = new mockFn();
5  const b = new mockFn();
6
7  mockFn.mock.instances[0] === a;
8  mockFn.mock.instances[1] === b;
9});