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Mocking modules in Jest.js

Authors
  • avatar
    Name
    Saad Bash

Mocking modules in Jest is important because it allows us to isolate and test specific parts of code without depending on external modules.

And when we are writing unit tests, we want to test only the code that we have written, not the external dependencies because we want to ensure that the code is working correctly and not impacted by external factors.

Mocking allows us to simulate the behavior of the module and control its responses to input & output.

First things first

When we require module in tests, calling jest.mock('./moduleName') is required.

CommonJS

Mocking a named import

// sum.js
module.exports = {
  sum: (a, b) => a + b,
}

// sum.test.js
const { sum } = require('./Module')

jest.mock('./Module', () => ({
  sum: () => 'mocked implementation',
}))

test('should use the mocked implementation', () => {
  expect(sum()).toEqual('mocked implementation')
})

Result when the test is run:
 PASS  ./Module.test.js
  ✓ should use mocked implementation (2 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.163 s, estimated 1 s
Ran all test suites.

In the above example, the sum function from Module.js is getting mocked inside the test file by passing a callback to jest.mock.

For default export, jest.mock syntax will look slightly different.

Default Export: Sum.js

// sum.js
const sum = (a, b) => a + b
module.exports = sum

// sum.test.js
const sum = require('./Sum')

jest.mock('./Sum', () => () => 'mocked implementation')

test('should use the mocked implementation', () => {
  expect(sum()).toEqual('mocked implementation')
})

And will get the same result in the terminal:

 PASS  ./Sum.test.js
  ✓ should used the mocked implementation (1 ms)

Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        0.11 s, estimated 1 s
Ran all test suites matching /Sum.test.js/i.

ES Module syntax

Mocking a named import

// hello.js
export function sayHello(name) {
  return `Hello, ${name}!`
}

// hello.test.js
import { sayHello } from './hello'

jest.mock('./hello', () => ({
  sayHello: (name) => `mocked`,
}))

test('Mock implementation test', () => {
  expect(sayHello()).toBe(`mocked`)
})

/*
PASS ./sum.test.js
✓ Mock implementation test

Test Suites: 1 passed, 1 total
Tests: 1 passed, 1 total
Snapshots: 0 total
Time: 0.307 s, estimated 1 s
*/

Mocking default import

import getWeather from './weather'

// weather.test.js
jest.mock('./weather', () => () => 'Sunny 47F')

Mocking only the named import (and leaving other imports unmocked)

import { getWeather, getTemperature, getWindSpeed } from './weather'

// weather.test.js
jest.mock('./weather', () => ({
  ...jest.requireActual('./weather'),
  getTemperature: () => 25,
  // getWeather and getWindSpeed will return their true values
}))

Mocking a default and named imports

import getCityName, { getPopulation } from './city'

jest.mock('./city', () => ({
  __esModule: true,
  default: () => 'New York City',
  getPopulation: () => 8_550_405,
}))
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