how to mock a library in jest

1// lodash.js
2const _ = require('lodash')
3module.exports = () => _.repeat('abc', 3)
4
5// lodash.test.js
6jest.mock('lodash')
7const lodashMock = require('lodash')
8const { repeat } = jest.requireActual('lodash')
9const lodashRepeat = require('../lodash')
10
11describe('manual mock module', () => {
12
13  beforeEach(() => {
14    // setup mock
15    lodashMock.repeat.mockReturnValue(repeat('xxx', 3))
16  })
17
18  it('lodash repeat value', () => {
19
20    // expect lodashMock
21    expect(jest.isMockFunction(lodashMock)).toBeTruthy()
22
23    // expect lodash.js
24    expect(lodashRepeat()).toBeDefined()
25    expect(lodashRepeat()).toBe("xxxxxxxxx")
26    expect(lodashRepeat().length).toBe(9)
27  })
28})
29

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();