Mock a function from another file - Jest

Question

I am writing Unit test cases for my application. There is one function which is written in Utils section and Used in all files. I wanted to mock this Utils function whenever I need but I am unable to do so.

Here is my code setup:

Utils.js

> const getData = (name) => "Hello !!! " + name; >  > const getContact = ()=> return Contacts.mobile; >  > export { >     getData, >     getContact } 

Login.js (Which uses Utils.js)

    const welcomeMessage = (name) => {      return getData(name);     } 

My Test file (Login.spec.js)

import { getData } from '../../src/utils';   jest.mock('getData', () => jest.fn())   describe('User actions', () => {      it('should get username', () => {         const value = 'Hello !!! Jest';         expect(welcomeMessage('Jest')).toEqual(value);     });  }); 

When I run my test case then I am getting this error:

 Cannot find module 'getData' from 'Login.spec.js' 

I tried to find the solution on official Jest Documentation and on SO as well but couldn't found anything. I am unable to fix this error and mock this function.

Answer 1

The first argument of jest.mock(...) must be a module path:

jest.mock('../../src/utils'); 

because the utils module is your code, not a 3rd lib, so you must learn manual mock of jest: https://facebook.github.io/jest/docs/en/manual-mocks.html

if you had this file: src/utils.js

you can mock it by creating a file: src/__mocks__/utils.js

content of this file is the replication of the original but replace the implementation by getData = jest.fn()

on you test file, just call: jest.mock('../../src/utils'); at begin of file.

then when you're familiar with, you can call that function inside beforeEach() and call its counter jest.unmock('../../src/utils'); insider afterEach()

An easy way to think about it is that:

when you call jest.mock('../../src/utils');, it means you tell jest that:

hey if the running test meets the line require('../../src/utils'), don't load it, let load ../../src/__mocks__/utils.

Answer 2

I got the same question and finally I find the solution. I post it here for the one who got the same issue.

Jest test file:

import * as utils from "./demo/utils"; import { welcomeMessage } from "./demo/login";  // option 1: mocked value const mockGetData = jest.spyOn(utils, "getData").mockReturnValue("hello");  // option 2: mocked function const mockGetData = jest.spyOn(utils, "getData").mockImplementation((name) => "Hello !!! " + name);  describe("User actions", () => {     it("should get username", () => {         const value = "Hello !!! Jest";         expect(welcomeMessage("Jest")).toEqual(value);     }); }); 

references: https://jestjs.io/docs/jest-object#jestfnimplementation

jest.spyOn(object, methodName)

Creates a mock function similar to jest.fn but also tracks calls to object[methodName]. Returns a Jest mock function.

Note: By default, jest.spyOn also calls the spied method. This is different behavior from most other test libraries. If you want to override the original function, you can use:

jest.spyOn(object, methodName).mockImplementation(() => customImplementation) 

or

object[methodName] = jest.fn(() => customImplementation);