How to do Unit Testing with Jest in React?

In this tutorial, we will create a new project to illustrate Test Driven Development (TDD) with React and Jest. We will create a simple model that can generate a greeting and integrate it in a Reach application. 

Part 1 – Creating the project

Let’s create a new project.

1. Create a directory C:\LabWork , open a command project.

2. Change into the LabWork directory [create the folder if required]:

cd C:\LabWork

3. Create a new React project using the following command:

npx create-react-app testing-app

4. Enter the new project:

cd testing-app

If desired, start your favorite text editor.

Part 2 – Creating our first test

In the spirit of TDD, let’s create a unit test to verify that our future model works correctly. The unit test will also serve as a specification for our model.

1. Using your preferred text editor, create the file src/Greeter.test.js

2. Enter the following code:

import React from 'react';
import Greeter from './Greeter'

test('provides a greeting', () => {
  const greeter = new Greeter('Ada Lovelace');

  const greeting = greeter.getGreeting();

  expect(greeting).toBe('Hello, Ada Lovelace')

3. Save your work.

4. Run the unit test and watch it fail:

npm run test

By default, the test running executes any files in the src directory that end with ‘.test.js’

5. Leave the test running in the command prompt window.

6. Create a new file called src/Greeter.js

7. Enter the following code:

export default class Greeter {
  constructor(userName = '') {
    this.userName = userName;

  getGreeting() {
    if (this.userName) {
      return `Hello, ${this.userName}`;
    } else {
      return `Hello, Anonymous`;

  async getGreetingAsync() {
    return this.getGreeting();

8. Save your work.

9. Return to the command prompt. Observer that the test now passes.

(If the test fails, verify your code from step 7)

10. Let’s complete the unit test. Add the following code at the end of src/Greeter.test.js:

test('provides a default greeting', () => {
  const greeter = new Greeter();

  const greeting = greeter.getGreeting();

  expect(greeting).toBe('Hello, Anonymous')

test('Can generate a greeting asynchronously', async () => {
  const greeter = new Greeter('Ada Lovelace');

  const greeting = await greeter.getGreetingAsync();

  expect(greeting).toBe('Hello, Ada Lovelace');

11. Save the file.

12. Return to the command prompt window. Observe that now all tests are said to pass.

Part 3 – Creating the React application

In the previous part, we performed unit tests but we did not write any React code. Now, let’s add a user interface on top of our model and test it using Jest.

1. Keep the current command prompt window open.

2. Start a second command prompt.

3. Change directory to C:\LabWork\testing-app

cd C:\LabWork\testing-app

4. Start the development server by executing the following:

npm run start

The placeholder React page comes up in the default browser.

5. Create the file src/GreeterComponent.js with the following code:

import React from 'react';
import Greeter from './Greeter'

export default class GreeterComponent extends React.Component {
  constructor(props) {

    this.state = {
      greeter: new Greeter(),

  static getDerivedStateFromProps(props, state) {
    return {
      greeter: new Greeter(props.userName)

  render() {
    return <div id='greet'>Message: {this.state.greeter.getGreeting()}</div>

6. Save the file.

7. Open src/App.js and replace the placeholder code with the following:

import React from 'react'
import './App.css'
import Greeter from './GreeterComponent'

export default class App extends React.Component {
  constructor(props) {
    this.state = {
      userName: '',

  updateUserName = (event) => {

  clearName = (event) => {
      userName: '',

  render() {
    return (
      <div id='app'>
          <label htmlFor='userName'>User Name: </label>
          <input type='text' id='userName' value={this.state.userName} onChange={this.updateUserName}/>
          <button onClick={this.clearName}>Clear</button>
        <Greeter userName={this.state.userName}/>

8. Save the file.

9. Return to the default web browser, and ensure the following page comes up:

10. Test the basic behavior by entering a name and observe the message down below. Test the clear button.

11. Return to the command-prompt that is running Jest. Notice there are src/App.test.js is now failing.

We are ready to add React tests!

Part 4 – Adding React Test

In this part, we will add automated test to ensure that our application renders as expected.

1. Open src/App.test.js

2. Replace the code with the following:

import React from 'react';
import { render } from '@testing-library/react';
import App from './App';
import { fireEvent } from '@testing-library/react'

test('renders two message elements', () => {
  const app = render(<App />);
  const inputHeadings = app.getAllByText(/message:/i);

3. Save the file.

4. Ensure that all unit test pass.

5. Let’s add a couple more unit test. Add the following code at the end of src/App.test.js:

test('has a GreetingComponent', () => {
  const app = render(<App />);

  const greetingComponent = app.getByText(/Anonymous/i);


test('changing the input updates the message', () => {
  const app = render(<App />);

  const input = app.getByLabelText(/user name/i);

  fireEvent.change(input, { target: { value: 'Ada Lovelace '}});

  const message1 = app.getByText(/Hello, Ada Lovelace/i);

  const clearButton = app.getByText(/clear/i);;

  const message2 = app.getByText(/Hello, Anonymous/i);

6. Save the file.

7. Ensure that all test pass.

8. Quit Jest by pressing ‘q’ at the command prompt.

9. Close all.

Part 5 – Review

In this tutorial, we created unit test to ensure our business logic and our rendering is correct. This will allow up to evolve the application with the confidence that and side-effect introduced by changes in the application can be quickly detected.

Angular 2 Programming Languages

Programming Languages for Angular Development

Angular 2 differs from Angular JS in regards to programming language support. With Angular JS you generally program in JavaScript. With Angular 2 the official site provides example code in several languages; JavaScript, TypeScript and Dart.

JavaScript is an implementation of the ECMAScript standard. Several versions exist including ES5 and ES6. Most current browsers fully support JavaScript ES5. This means that code compliant with ES5 will run on most most browsers without modification. ES6 on the other hand may not be supported and is typically converted to ES5 before it gets to the browser. The more recent version of JavaScript (ES6) adds features and makes certain tasks easier. It is not clear when browsers will be fully ES6 compliant.

TypeScript and Dart are language super sets of JavaScript. This means they include all the features of JavaScript but add many helpful features as well. But just like the ES6 version of JavaScript they don’t work on current browsers. To work around this limitation there are utilities to convert code written in their non-browser-compliant syntax back to JavaScript ES5. In this way they provide the developer with advanced features and still allow code to be run on existing browsers. The process of converting code from one language to another is referred to as ‘transpilation’. Technically this conversion is performed by a language pre-processor that, although it does not compile anything, is commonly referred to as a compiler.

TypeScript is an open source language that is developed and maintained by Microsoft. It provides type safety, advanced object oriented features, and simplified module loading. Angular 2 itself is built using TypeScript. The documentation support is better on the Angular 2 site for TypeScript than it is for the other options. So although you could write your Angular 2 apps in pure ES5 JavaScript you may find it more difficult and find less support if you get into trouble than if you bite the bullet and spends some time learning TypeScript.

Dart is another alternative for programming Angular 2 applications. It is similar in features to TypeScript and has its own loyal following. If your team is already versed in Dart and is looking to start developing in Angular 2 it could be a good choice. That being said,  in the rest of this post, we will be concentrating on TypeScript.

Code Example

A basic Angular 2 component coded in TypeScript:

import { Component } from '@angular/core';
selector: 'hello-app',
template: '<h1>Hello Angular 2</h1>'
export class AppComponent { }


The same component coded in JavaScript ES5:

(function(app) {
app.AppComponent = ng.core.Component({
selector: 'hello-app',
template: '<h1>Hello Angular 2</h1>'
constructor: function() {}
})( || ( = {}));

As you can see the TypeScript version is more concise and easy to read.


TypeScript Usage Basics

TypeScript support is provided by the Node.js ‘typescript’ package. To use typescript you first need to load Node.js and the Node Package Manager (npm). Once Node.js is installed on you system you would use the following command to install TypeScript support:

npm install -g typescript

This will install the TypeScript compiler and allow you to run it from the command line using the following command:


There are generally three methods to execute tsc. The first method involves passing the name of the TypeScript code file you want compiled on the command line:

tsc filename.ts

Here’s a simple TypeScript code file:

// app.ts
var msg:string = "Hello Typescript!";
function getMessage(message: string){
return message;

Compiling this file produces a JavaScript file with the same name as the TypeScript file but with an extension of “.js”.

tsc app.ts
(produces app.js)

You can test the file by running it using Node.js like this:

node app.js

As your application gets more complex and you have multiple *.ts files in one or more directories you may want to take advantage of the second method of running tsc. To do this you will first need to set up a tsconfig.json config file. Among other things this file tells tsc which directories contain the files you wish to compile. After placing this file in the root of your project tsc will compile any *.ts files it finds whenever they are modified and saved. To use this method you launch tsc in a terminal window and then edit your *.ts files in any text editor. As you save your files they will automatically be converted to JavaScript.

The third method involves using a text editor plugin to determine when you have saved a *.ts file. When that happens the editor itself will call tsc to perform the conversion. This kind of plugin is available for the Atom text editor. Microsoft Visual Studio also includes this type of support. For those who don’t want to load the complete Visual Studio development system Microsoft’s Visual Studio Core text editor also includes TypeScript support.


Using TypeScript significantly simplifies the code you need to write when creating Angular 2 applications. Although TypeScript can’t be executed by browsers directly there are several ways to convert TypeScript into code that browsers can work with.

New Support for ECMAScript 2015

Although it is a huge part of the modern web, the syntax of JavaScript hasn’t really changed significantly in quite a long time. That all changed though last June, when ‘ECMAScript 2015’ was released.  This major update contained some significant changes and new features.  Now that more JavaScript engines and environments support the new specification it is a good time to get familiar with these changes.

Besides browsers adding more support for ECMAScript 2015, also call ECMAScript 6 or just “ES 6”, several of the popular JavaScript platforms are adding support as well.  Node.js 4 & 5 support a majority of ES 6, as does AngularJS 2.0.  Of course, like any major version upgrade of such a fundamental web technology, there will be a gradual migration.  We will see a lot of the same things happen with JavaScript that happened with HTML5 and CSS 3, both of which are now “mainstream” and enjoy much broader support.

One thing we’ve learned at Web Age is you can’t wait for “100% support in 100% of environments” to learn about or even begin to adopt a new technology.  It is important to learn about such major upgrades now so that you are aware what is different and look for places where a new version of a technology might be leveraged.

For those looking to keep up with these major changes to JavaScript introduced in ECMAScript 2015, we have a few resources.  The primary one is a new one-day course that focuses on the changes introduced with ECMAScript 2015.  We chose to release a class focused only on the changes since there are lots of people that have JavaScript skills and just need to learn about the changes.  You can find that course here:

WA2488 JavaScript Changes with ECMAScript 2015

We also are going to have a webinar this week that will provide a very quick overview of the changes.  This webinar will cover the main differences with the new ECMAScript 2015 specification and the impact on how JavaScript code can be written.  In particular, focus will be paid on how various ways to leverage the new version while also considering backwards compatibility.  You can register for that webinar here:

WEBINAR: Changes in ECMAScript 2015

As with all new and emerging technologies, we at Web Age look forward to keeping you up to date on what is happening and helping you learn about these technologies to expand the scope of your development skills!