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JS tests: mocking best practices

You’ll be able to mock imports in a number of methods, all of them legitimate, nevertheless it’s normally greatest to make use of jest.mock() for “static” mocks and jest.spyOn() for mocks you should change the implementation for particular take a look at circumstances.
There’s additionally some world configuration and take a look at file construction which may also help us with that and to verify our exams are unbiased and constant.

Assessments are normally the forgotten ones on the subject of greatest practices. They’re getting an identical remedy as CSS structure or HTML accessibility, forgotten till you should care about them.

It is also a undeniable fact that JS testing (and specifically within the Frontend) is kind of current but, and I am certain this may enhance with time, however the actuality is that almost all firms nonetheless do not add exams to their JS codebases, and in the event that they do, they’re normally a large number.

In JS you’ll be able to normally obtain the identical lead to a number of methods, however that does not imply all of them are equal. That is why I consider greatest practices are much more essential in our ecosystem.

Mocks are most likely the toughest half to maintain organised and clear in your take a look at suite. However there are some choices you may make that can assist you with that.

The very first thing you should know is what sort of take a look at you’re writing. I normally seek advice from this article by Kent C. Dodds, one in every of (if not probably the most) related actors within the Javascript testing group. Relying on the type of take a look at, the info you are mocking and the way you are doing it ought to change.

On this article I will give some examples written with Jest, because it’s nonetheless the most well-liked JS take a look at runner on the market, however word that should you’re utilizing Vitest (and also you most likely ought to) virtually all of the syntax is identical, so this is applicable too.

Neither Jest nor Vitest clear, reset or restore your mocks by default after every take a look at. That is very opinionated, however imho that should not be the default.

You need your exams to be constant and unbiased from one another, so a take a look at shouldn’t depend on a earlier take a look at to move or fail.

Let’s have a look at what means to clear, reset and restore a mock, based mostly on Jest’s configuration docs:

  • clearMocks: Mechanically clear mock calls, cases, contexts and outcomes earlier than each take a look at. Equal to calling jest.clearAllMocks() earlier than every take a look at. This doesn’t take away any mock implementation that will have been supplied.
  • resetMocks: Mechanically reset mock state earlier than each take a look at. Equal to calling jest.resetAllMocks() earlier than every take a look at. This may result in any mocks having their faux implementations eliminated however doesn’t restore their preliminary implementation.
  • restoreMocks: Mechanically restore mock state and implementation earlier than each take a look at. Equal to calling jest.restoreAllMocks() earlier than every take a look at. This may result in any mocks having their faux implementations eliminated and restores their preliminary implementation.

So not activating clearMocks in your world config means a mocked perform will hold all of its calls from earlier exams, so you possibly can simply get to a false constructive/unfavourable when asserting a mocked perform has been known as or not, and a take a look at may even move once we run the entire take a look at suite however fail if we run it alone. We do not need that, so clearMocks ought to be set to true within the world config.

With reference to resetMocks and restoreMocks, it isn’t so straight ahead. If we actually need a clear state earlier than every take a look at, we must always allow restoreMocks, since that’s the just one that restores the unique implementation. The caveat is that if we mock one thing for one take a look at, we normally want that to be mocked for all exams in that take a look at suite, perhaps simply with completely different mock implementations/return values.

Even there are some exceptions, I like to recommend setting restoreMocks to true, as a result of that is the one manner you may make certain every take a look at is unbiased and that may permit you to obtain a extra sustainable take a look at suite. However that is a powerful alternative, so you may have to adapt the way in which you organise and set your mocks to keep away from code duplicity hell in every take a look at.

Fortunately, there is a strategy to have one of the best of each worlds. Jest has beforeEach, afterEach, beforeAll and afterAll strategies that, at the side of a superb take a look at construction, it will probably be sure to have the specified mocks for each take a look at. However we’ll come again to this later.

As I mentioned, there are many methods you’ll be able to mock issues in JS with Jest or Vitest, however though you’ll be able to obtain an identical end result with most of them it doesn’t suggest you must use them indistinctly.

Once more, as per Jest docs:

  • jest.fn(implementation?): returns a brand new, unused mock perform. Optionally takes a mock implementation.
  • jest.spyOn(object, methodName): creates a mock perform just like jest.fn but in addition tracks calls to object[methodName]. Returns a Jest mock perform.
  • jest.mock(moduleName, manufacturing unit, choices): mocks a module with an auto-mocked model when it’s being required. manufacturing unit and choices are non-compulsory.

When to make use of each?

I will not argue what ought to or should not be mocked on this article, that is an entire completely different subject. Normally we’ll be utilizing this utilities to switch the behaviour of a perform as a result of we do not need it to have an effect on the take a look at end result. That is helpful when testing code that depends on different utilities, third get together libraries or Backend endpoints.


That is the one we’ll use most incessantly. We’ll normally use it to mock perform parameters (or perform props if we’re testing parts).


We must always use this one once we need to mock an imported technique and we would like completely different mocked implementations/return values relying on the take a look at. For instance, you need to take a look at each the success and error stream after an endpoint name response, so your mock ought to resolve and throw respectively.


Whenever you need to mock a full imported module or some components and also you need it to have the identical behaviour throughout all exams in that file, you may use jest.mock. It is beneficial to incorporate jest.requireActual within the mock since that’ll be leaving the remainder of the module that you just’re not explicitly mocking with its unique implementation.

However why?

You could possibly undoubtedly use jest.spyOn and jest.mock each for constant and altering implementations respectively, however I consider that this fashion makes extra sense, and you will get it once we get to the final part of this text.
The principle aim is to attain a nicely organised take a look at construction, and we have to make some choices to get there.
If you wish to change the implementation/return worth of an imported perform mocked with jest.mock, you must beforehand declare a variable, assign it a jest.fn with the “default” implementation after which move it to jest.mock. After that, you may have to seek advice from that variable within the particular take a look at you need to change its implementation, so it makes it a bit extra verbose and makes your top-level mocks readability a bit extra complicated.
Apart from, jest.spyOn means that you can solely mock a particular aspect of the exported module with out having to fret about overwriting the remainder of the module exported components.

You’ll be able to assume this isn’t related, I although it too not so way back, however this may also help you wrap every little thing talked about earlier than and make your exams extra readable, sustainable and constant. It is just like the cherry on high the place every little thing is smart.

If you happen to’ve been writing exams you may already know we now have describe and it blocks, the primary one is used to group exams and the second to outline a particular take a look at case.

We’ll attempt to use the describe blocks to construction our exams taking into account the completely different eventualities the place we have to take a look at our code. Meaning we’ll be utilizing them to set mock implementations that will probably be shared throughout all take a look at circumstances inside that block, and we are able to use the beforehand talked about beforeEach technique to attain that.

Present me the code

Let me write an instance. Think about we now have a the next perform:

import { irrelevantMethod } from 'module-1';
import { getSomeThings } from 'module-2';

export perform getArrayOfThings(quantity) {
  if (!quantity) {
    return [];
  const end result = getSomeThings();
  if (!end result) return [];
  return end result.slice(0, Math.min(quantity, 4));
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Our perform has some dependencies, that are:

  • irrelevantMethod: it is a technique our perform has to name however that does not have an effect on the end result in any manner (therefore its identify). An actual-life instance of this may very well be occasion monitoring.
  • getSomeThings: this technique does have an effect on the results of our perform, so we’ll be mocking it and altering its mocked return worth in a few of our exams. We’re assuming we all know this technique can solely return null or a sound array of a set appreciable size.

If we put every little thing we have seen collectively, the take a look at file for this technique may look one thing like this:

import * as module2 from 'module-2';
import { getArrayOfThings } from '../utils.js';

const mockedIrrelevantMethod = jest.fn();
jest.mock(() => ({
  irrelevantMethod: mockedIrrelevantMethod,

describe('getArrayOfThings', () => {
  it('ought to return an empty array if quantity is 0', () => {
    const end result = getArrayOfThings(0);
    anticipate(end result).toEqual([]);

  it('ought to name irrelevantMethod and getSomeThings if quantity is bigger than 0', () => {
    const mockedGetSomeThings = jest.spyOn(module2, 'getSomeThings');

  describe('getSomeThings returns null', () => {
    let mockedGetSomeThings;
    beforeEach(() => {
      mockedGetSomeThings = jest.spyOn(module2, 'getSomeThings').mockReturnValue(null);

    it('ought to return an empty array', () => {
      const end result = getArrayOfThings(1);
      anticipate(end result).toEqual([]);

  describe('getSomeThings returns an array', () => {
    let mockedGetSomeThings;
    beforeEach(() => {
      mockedGetSomeThings = jest.spyOn(module2, 'getSomeThings').mockReturnValue([1, 2, 3, 4, 5, 6]);

    it('ought to return an array of "quantity" components if "quantity" is 4 or much less', () => {
      const end result = getArrayOfThings(3);
      anticipate(end result).toEqual([1, 2, 3]);

    it('ought to return an array of 4 components if "quantity" is bigger than 4', () => {
      const end result = getArrayOfThings(5);
      anticipate(end result).toEqual([1, 2, 3, 4]);
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It is a quite simple instance, typically you may have to have a number of describe ranges, each with its beforeEach callback that may have an effect on all of the exams in that block. That is nice, and can truly aid you hold your exams much more readable.

It is a very opinionated manner of organising exams and mocks, however after a few years testing javascript code, it is by far the method that works greatest for me.

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