Jest mock module multiple times with different values

Question

I have a function that I want to test and this function uses an imported module:

var a = require('./a');

function add(b) {
  return a + b;
}

module.exports = add;

That a module returns a number in this sample, but in my real project I use that as a config object that is changed from time to time manually.

var a = 1;
module.exports = a;

The test for the add function looks like this:

describe('add', () => {

  it('should add the mock number 1 to 2', () => {
    jest.setMock('./a', 1);
    const add = require('./add');
    expect(add(2)).toBe(3);
  });

  it('should add the mock number 2 to 2', () => {
    jest.setMock('./a', 2);
    const add = require('./add');
    expect(add(2)).toBe(4);
  });
});

First test passes, The second test fails because it inherits from the first mock. Is there any way to mock the a module multiple times?

I would like a solution that doesn't imply refactoring the add function and instead focus on mocking that module multiple times. (in my real project that is a config file)

You can play around with the code here: https://repl.it/@adyz/NocturnalBadComma

Answer 1

Add

beforeEach(() => {
    jest.resetModules();
});

Final tests

describe('add', () => {

    beforeEach(() => {
        jest.resetModules();
    });

    it('should add the mock number 5 to 2', () => {
        jest.setMock('./a', 5);
        const add = require('./add');
        expect(add(2)).toBe(7);
    });

    it('should add the mock number 2 to 2', () => {
        jest.setMock('./a', 2);
        const add = require('./add');
        expect(add(2)).toBe(4);
    });
});

Demo: https://repl.it/repls/TrustingBelatedProprietarysoftware

Answer 2

To add to @Gigi's solution, I created another example, using jest.mock:

In the file multiplier.ts, multiplier is the exported function we want to test:

  // file: multiplier.ts

  import {getNumber} from './get-number'
  const multiplier = (num:number) => num * getNumber()
  export {multiplier}

In the file get-number.ts, getNumber is the module we want to mock:

  // file: get-number.ts

  const getNumber = () => 2
  export {getNumber}

Here is the test:

  // file: multiplier.test.ts

  // import { multiplier } from "./multiplier"  // <-- this will not work
  
  describe("[multiplier]", () => {
    beforeEach(() => {
      jest.resetModules()
    })
  
    it('should mock getNumber so that getNumber return 3', () => {
      const mockReturn = 3
  
      jest.mock( './get-number', () => (
        { getNumber: jest.fn(()=>mockReturn) }
      ))
  
      const { multiplier } = require('./multiplier')
      expect(multiplier(2)).toBe(6)
    })
  
    it('should mock getNumber so that getNumber return 4', () => {
      const mockReturn = 4
  
      jest.mock( './get-number', () => (
        { getNumber: jest.fn(()=>mockReturn) }
      ))
  
      const { multiplier } = require('./multiplier')
      expect(multiplier(2)).toBe(8)
    })
  
    it('should mock getNumber so that getNumber return 5', () => {
      const mockReturn = 5
  
      jest.mock( './get-number', () => (
        { getNumber: jest.fn(()=>mockReturn) }
      ))
  
      const { multiplier } = require('./multiplier')
      expect(multiplier(2)).toBe(10)
    })
  })

Note: for this to work, we need to use require to import multiplier.ts