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How you can Check React Parts Utilizing Jest – SitePoint

This text is by visitor writer Jack Franklin. SitePoint visitor posts purpose to convey you participating content material from outstanding writers and audio system of the JavaScript group.

On this article, we’ll check out utilizing Jest — a testing framework maintained by Fb — to check our React parts. We’ll have a look at how we are able to use Jest first on plain JavaScript capabilities, earlier than a few of the options it offers out of the field particularly aimed toward making testing React apps simpler.

It’s price noting that Jest isn’t aimed particularly at React: you should utilize it to check any JavaScript functions. Nonetheless, a few the options it offers are available actually useful for testing consumer interfaces, which is why it’s an ideal match with React.

Pattern Utility

Earlier than we are able to take a look at something, we’d like an utility to check! Staying true to internet growth custom, I’ve constructed a small todo utility that we’ll use as the start line. Yow will discover it, together with all of the assessments that we’re about to put in writing, on GitHub. For those who’d prefer to play with the appliance to get a really feel for it, you too can discover a reside demo on-line.

The appliance is written in ES2015, compiled utilizing webpack with the Babel ES2015 and React presets. I gained’t go into the main points of the construct setup, nevertheless it’s all within the GitHub repo in case you’d prefer to test it out. You’ll discover full directions within the README on learn how to get the app operating regionally. For those who’d prefer to learn extra, the appliance is constructed utilizing webpack, and I like to recommend “A Newbie’s information to webpack” as a great introduction to the device.

The entry level of the appliance is app/index.js, which simply renders the Todos element into the HTML:

  <Todos />,

The Todos element is the principle hub of the appliance. It incorporates all of the state (hard-coded knowledge for this utility, which in actuality would probably come from an API or related), and has code to render the 2 baby parts: Todo, which is rendered as soon as for every todo within the state, and AddTodo, which is rendered as soon as and offers the shape for a consumer so as to add a brand new todo.

As a result of the Todos element incorporates all of the state, it wants the Todo and AddTodo parts to inform it at any time when something modifications. Subsequently, it passes capabilities down into these parts that they will name when some knowledge modifications, and Todos can replace the state accordingly.

Lastly, for now, you’ll discover that each one the enterprise logic is contained in app/state-functions.js:

export operate toggleDone(todos, id) {}

export operate addTodo(todos, todo) {}

export operate deleteTodo(todos, id) {}

These are all pure capabilities that take the state (which, for our pattern app, is an array of todos) and a few knowledge, and return the brand new state. For those who’re unfamiliar with pure capabilities, they’re capabilities that solely reference knowledge they’re given and don’t have any negative effects. For extra, you’ll be able to learn my article on A Record Aside on pure capabilities and my article on SitePoint about pure capabilities and React.

For those who’re acquainted with Redux, they’re pretty just like what Redux would name a reducer. The truth is, if this utility bought a lot larger I might think about shifting into Redux for a extra specific, structured method to knowledge. However for an utility this dimension, you’ll usually discover that native element state and a few effectively abstracted capabilities will likely be greater than sufficient.

To TDD or To not TDD?

There have been many articles written on the professionals and cons of test-driven growth, the place builders are anticipated to put in writing the assessments first, earlier than writing the code to repair the take a look at. The concept behind that is that, by writing the take a look at first, it’s important to take into consideration the API you’re writing, and it could result in a greater design. I discover that this very a lot comes down to non-public desire and likewise to the form of factor I’m testing. I’ve discovered that, for React parts, I like to put in writing the parts first after which add assessments to crucial bits of performance. Nonetheless, in case you discover that writing assessments first on your parts suits your workflow, then it’s best to do this. There’s no exhausting rule right here; do no matter feels finest for you and your workforce.

Introducing Jest

Jest was first launched in 2014, and though it initially garnered a number of curiosity, the mission was dormant for some time and never so actively labored on. Nonetheless, Fb has invested a number of effort into enhancing Jest, and not too long ago revealed a number of releases with spectacular modifications that make it price reconsidering. The one resemblance of Jest in comparison with the preliminary open-source launch is the identify and the brand. Every little thing else has been modified and rewritten. For those who’d like to seek out out extra about this, you’ll be able to learn Christoph Pojer’s remark, the place he discusses the present state of the mission.

For those who’ve been annoyed by organising Babel, React and JSX assessments utilizing one other framework, then I undoubtedly advocate giving Jest a attempt. For those who’ve discovered your present take a look at setup to be sluggish, I additionally extremely advocate Jest. It routinely runs assessments in parallel, and its watch mode is ready to run solely assessments related to the modified file, which is invaluable when you’ve a big suite of assessments. It comes with JSDom configured, that means you’ll be able to write browser assessments however run them by Node. It will probably take care of asynchronous assessments and has superior options resembling mocking, spies and stubs in-built.

Putting in and Configuring Jest

To begin with, we have to get Jest put in. As a result of we’re additionally utilizing Babel, we’ll set up one other couple of modules that make Jest and Babel play properly out of the field, together with Babel and the required presets:

npm set up --save-dev jest babel-jest @babel/core @babel/preset-env @babel/preset-react

You additionally have to have a babel.config.js file with Babel configured to make use of any presets and plugins you want. The pattern mission already has this file, which seems like this:

module.exports = {
  presets: [

This text gained’t be going into depth on organising Babel. I like to recommend the Babel utilization information in case you’d prefer to be taught extra about Babel particularly.

We gained’t set up any React testing instruments but, as a result of we’re not going to start out with testing our parts, however our state capabilities.

Jest expects to seek out our assessments in a __tests__ folder, which has turn out to be a preferred conference within the JavaScript group, and it’s one we’re going to stay to right here. For those who’re not a fan of the __tests__ setup, out of the field Jest additionally helps discovering any .take a look at.js and .spec.js recordsdata too.

As we’ll be testing our state capabilities, go forward and create __tests__/state-functions.take a look at.js.

We’ll write a correct take a look at shortly, however for now, put on this dummy take a look at, which is able to allow us to examine all the pieces’s working appropriately and we have now Jest configured:

describe('Addition', () => {
  it('is aware of that 2 and a couple of make 4', () => {
    anticipate(2 + 2).toBe(4);

Now, head into your bundle.json. We have to arrange npm take a look at in order that it runs Jest, and we are able to do this just by setting the take a look at script to run jest:

"scripts": {
  "take a look at": "jest"

For those who now run npm take a look at regionally, it’s best to see your assessments run, and cross!

PASS  __tests__/state-functions.take a look at.js
    ✓ is aware of that 2 and 2 make 4 (5ms)

Check Suites: 1 handed, 1 whole
Assessments:       1 handed, 1 whole
Snapshots:   0 handed, 0 whole
Time:        3.11s

For those who’ve ever used Jasmine, or most testing frameworks, the above take a look at code itself ought to be fairly acquainted. Jest lets us use describe and it to nest assessments as we have to. How a lot nesting you utilize is as much as you. I prefer to nest mine so all of the descriptive strings handed to describe and it learn virtually as a sentence.

In the case of making precise assertions, you wrap the factor you need to take a look at inside an anticipate() name, earlier than then calling an assertion on it. On this case, we’ve used toBe. Yow will discover an inventory of all of the out there assertions within the Jest documentation. toBe checks that the given worth matches the worth underneath take a look at, utilizing === to take action. We’ll meet a number of of Jest’s assertions by this tutorial.

Testing Enterprise Logic

Now that we’ve seen Jest work on a dummy take a look at, let’s get it operating on an actual one! We’re going to check the primary of our state capabilities, toggleDone. toggleDone takes the present state and the ID of a todo that we’d prefer to toggle. Every todo has a accomplished property, and toggleDone ought to swap it from true to false, or vice-versa.

Observe: in case you’re following together with this, ensure you’ve cloned the repo and have copied the app folder to the identical listing that incorporates your ___tests__ folder. You’ll additionally want to put in all of the app’s dependencies (resembling React). You may make positive it’s all put in by operating npm set up when you’ve cloned the repository.

I’ll begin by importing the operate from app/state-functions.js, and organising the take a look at’s construction. While Jest lets you use describe and it to nest as deeply as you’d prefer to, you too can use take a look at, which is able to usually learn higher. take a look at is simply an alias to Jest’s it operate, however can typically make assessments a lot simpler to learn and fewer nested.

For instance, right here’s how I might write that take a look at with nested describe and it calls:

import { toggleDone } from '../app/state-functions';

describe('toggleDone', () => {
  describe('when given an incomplete todo', () => {
    it('marks the todo as accomplished', () => {

And right here’s how I might do it with take a look at:

import { toggleDone } from '../app/state-functions';

take a look at('toggleDone completes an incomplete todo', () => {

The take a look at nonetheless reads properly, however there’s much less indentation getting in the way in which now. This one is especially down to non-public desire; select whichever fashion you’re extra snug with.

Now we are able to write the assertion. First, we’ll create our beginning state, earlier than passing it into toggleDone, together with the ID of the todo that we need to toggle. toggleDone will return our end state, which we are able to then assert on:

import { toggleDone } from "../app/state-functions";

take a look at("tooggleDone completes an incomplete todo", () => {
  const startState = [{ id: 1, done: false, text: "Buy Milk" }];
  const finState = toggleDone(startState, 1);

  anticipate(finState).toEqual([{ id: 1, done: true, text: "Buy Milk" }]);

Discover now that I exploit toEqual to make my assertion. You must use toBe on primitive values, resembling strings and numbers, however toEqual on objects and arrays. toEqual is constructed to take care of arrays and objects, and can recursively examine every subject or merchandise throughout the object given to make sure that it matches.

With that, we are able to now run npm take a look at and see our state operate take a look at cross:

PASS  __tests__/state-functions.take a look at.js
  ✓ tooggleDone completes an incomplete todo (9ms)

Check Suites: 1 handed, 1 whole
Assessments:       1 handed, 1 whole
Snapshots:   0 handed, 0 whole
Time:        3.166s

Rerunning Assessments on Modifications

It’s a bit irritating to make modifications to a take a look at file after which should manually run npm take a look at once more. One among Jest’s finest options is its watch mode, which watches for file modifications and runs assessments accordingly. It will probably even work out which subset of assessments to run based mostly on the file that modified. It’s extremely highly effective and dependable, and also you’re in a position to run Jest in watch mode and go away all of it day while you craft your code.

To run it in watch mode, you’ll be able to run npm take a look at -- --watch. Something you cross to npm take a look at after the primary -- will likely be handed straight by to the underlying command. Because of this these two instructions are successfully equal:

  • npm take a look at -- --watch
  • jest --watch

I’d advocate that you just go away Jest operating in one other tab, or terminal window, for the remainder of this tutorial.

Earlier than shifting on to testing the React parts, we’ll write yet one more take a look at on one other one in all our state capabilities. In an actual utility I might write many extra assessments, however for the sake of the tutorial, I’ll skip a few of them. For now, let’s write a take a look at that ensures that our deleteTodo operate is working. Earlier than seeing how I’ve written it under, attempt writing it your self and seeing how your take a look at compares.

Keep in mind that you’ll should replace the import assertion on the prime to import deleteTodo together with toggleTodo:

import { toggleDone, deleteTodo } from "../app/state-functions";

And right here’s how I’ve written the take a look at:

take a look at('deleteTodo deletes the todo it's given', () => {
  const startState = [{ id: 1, done: false, text: 'Buy Milk' }];
  const finState = deleteTodo(startState, 1);


The take a look at doesn’t range an excessive amount of from the primary: we arrange our preliminary state, run our operate after which assert on the completed state. For those who left Jest operating in watch mode, discover the way it picks up your new take a look at and runs it, and the way fast it’s to take action! It’s an effective way to get on the spot suggestions in your assessments as you write them.

The assessments above additionally show the right structure for a take a look at, which is:

  • arrange
  • execute the operate underneath take a look at
  • assert on the outcomes

By holding the assessments specified by this manner, you’ll discover them simpler to comply with and work with.

Now that we’re pleased testing our state capabilities, let’s transfer on to React parts.

Testing React Parts

It’s price noting that, by default, I might really encourage you to not write too many assessments in your React parts. Something that you just need to take a look at very totally, resembling enterprise logic, ought to be pulled out of your parts and sit in standalone capabilities, similar to the state capabilities that we examined earlier. That stated, it’s helpful at occasions to check some React interactions (ensuring a particular operate known as with the appropriate arguments when the consumer clicks a button, for instance). We’ll begin by testing that our React parts render the appropriate knowledge, after which have a look at testing interactions.

To put in writing our assessments, we’ll set up Enzyme, a wrapper library written by Airbnb that makes testing React parts a lot simpler.

Observe: since this text was first written, the React workforce has shifted away from Enzyme and as a substitute recommends React Testing Library (RTL). It’s price having a learn of that web page. For those who’re sustaining a codebase that has Enzyme assessments already, there’s no have to drop all the pieces and transfer away, however for a brand new mission I’d advocate contemplating RTL.

Together with Enzyme, we’ll additionally want to put in the adapter for whichever model of React we’re utilizing. For React v16, this might be enzyme-adapter-react-16, however for React v17 there’s presently no official adapter out there, so we’ll have to make use of an unofficial model. Please be aware that this bundle is meant as a stop-gap till official help is launched and will likely be deprecated at the moment.

You possibly can comply with the progress on an official model in this GitHub subject.

npm set up --save-dev enzyme @wojtekmaj/enzyme-adapter-react-17

There’s a small quantity of setup that we’d like for Enzyme. Within the root of the mission, create setup-tests.js and put this code in there:

import { configure } from 'enzyme';
import Adapter from '@wojtekmaj/enzyme-adapter-react-17';

configure({ adapter: new Adapter() });

We then want to inform Jest to run this file for us earlier than any assessments get executed. We will do this by configuring the setupFilesAfterEnv possibility. You possibly can put Jest config in its personal file, however I like to make use of bundle.json and put issues inside a jest object, which Jest will even choose up:

"jest": {
  "setupFilesAfterEnv": [

Now we’re prepared to put in writing some assessments! Let’s take a look at that the Todo element renders the textual content of its todo inside a paragraph. First we’ll create __tests__/todo.take a look at.js, and import our element:

import Todo from '../app/todo';
import React from 'react';
import { mount } from 'enzyme';

take a look at('Todo element renders the textual content of the todo', () => {

I additionally import mount from Enzyme. The mount operate is used to render our element after which enable us to examine the output and make assertions on it. Although we’re operating our assessments in Node, we are able to nonetheless write assessments that require a DOM. It’s because Jest configures jsdom, a library that implements the DOM in Node. That is nice as a result of we are able to write DOM-based assessments with out having to fireside up a browser every time to check them.

We will use mount to create our Todo:

const todo = { id: 1, accomplished: false, identify: 'Purchase Milk' };
const wrapper = mount(
  <Todo todo={todo} />

After which we are able to name, giving it a CSS selector, to seek out the paragraph that we’re anticipating to comprise the textual content of the Todo. This API would possibly remind you of jQuery, and that’s by design. It’s a really intuitive API for looking out rendered output to seek out the matching parts.

const p ='.toggle-todo');

And eventually, we are able to assert that the textual content inside it’s Purchase Milk:

anticipate(p.textual content()).toBe('Purchase Milk');

Which leaves our total take a look at wanting like so:

import Todo from '../app/todo';
import React from 'react';
import { mount } from 'enzyme';

take a look at('TodoComponent renders the textual content inside it', () => {
  const todo = { id: 1, accomplished: false, identify: 'Purchase Milk' };
  const wrapper = mount(
    <Todo todo={todo} />
  const p ='.toggle-todo');
  anticipate(p.textual content()).toBe('Purchase Milk');

And now we have now a take a look at that checks we are able to render todos efficiently.

Subsequent, let’s have a look at how you should utilize Jest’s spy performance to say that capabilities are known as with particular arguments. That is helpful in our case, as a result of we have now the Todo element that’s given two capabilities as properties, which it ought to name when the consumer clicks a button or performs an interplay.

On this take a look at, we’re going to say that when the todo is clicked, the element will name the doneChange prop that it’s given:

take a look at('Todo calls doneChange when todo is clicked', () => {

We need to have a operate that we are able to use to maintain observe of its calls, and the arguments that it’s known as with. Then we are able to examine that, when the consumer clicks the todo, the doneChange operate known as and likewise known as with the proper arguments. Fortunately, Jest offers this out of the field with spies. A spy is a operate whose implementation you don’t care about; you simply care about when and the way it’s known as. Consider it as you spying on the operate. To create one, we name jest.fn():

const doneChange = jest.fn();

This provides a operate that we are able to spy on and ensure it’s known as appropriately. Let’s begin by rendering our Todo with the appropriate props:

const todo = { id: 1, accomplished: false, identify: 'Purchase Milk' };
const doneChange = jest.fn();
const wrapper = mount(
  <Todo todo={todo} doneChange={doneChange} />

Subsequent, we are able to discover our paragraph once more, similar to within the earlier take a look at:

const p =".toggle-todo");

After which we are able to name simulate on it to simulate a consumer occasion, passing click on because the argument:

p.simulate('click on');

And all that’s left to do is assert that our spy operate has been known as appropriately. On this case, we’re anticipating it to be known as with the ID of the todo, which is 1. We will use anticipate(doneChange).toBeCalledWith(1) to say this — and with that, we’re accomplished with our take a look at!

take a look at('TodoComponent calls doneChange when todo is clicked', () => {
  const todo = { id: 1, accomplished: false, identify: 'Purchase Milk' };
  const doneChange = jest.fn();
  const wrapper = mount(
    <Todo todo={todo} doneChange={doneChange} />

  const p ='.toggle-todo');
  p.simulate('click on');


Fb launched Jest a very long time in the past, however in latest occasions it’s been picked up and labored on excessively. It has quick turn out to be a favourite for JavaScript builders and it’s solely going to get higher. For those who’ve tried Jest up to now and never favored it, I can’t encourage you sufficient to attempt it once more, as a result of it’s virtually a unique framework now. It’s fast, nice at rerunning specs, provides incredible error messages, and has an ideal expressive API for writing good assessments.

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