How to Test React Applications with Jest and React Testing Library

Learn how to test React applications with Jest and React Testing Library, a popular combination and official recommendation from React.

Testing is an essential practice in software engineering. It helps build robust and high-quality software and boosts the team’s confidence in the code, making the application more flexible and prone to fewer errors when introducing or modifying features.

Highly efficient teams make testing a core practice in their everyday routines, and no feature is released before automated tests are in place. Some developers even write the tests before writing the features, following a process called TDD (Test Driven Development).

In this article, we’ll test React applications with Jest and React Testing Library, a popular combination of a JavaScript testing framework and a React utility for testing components. It’s also the official recommendation given on React’s documentation.

What is testing?

Testing is the process of automating assertions between the results the code produces and what we expect the results to be.

When testing React applications, our assertions are defined by how the application renders and responds to user interactions.

There are many different types of testing paradigms and philosophies. This article will focus on creating unit and component tests (or integration tests).

Introduction to Jest and React Testing Library

What is Jest?

Jest is a JavaScript testing framework that allows developers to run tests on JavaScript and TypeScript code and integrates well with React.

It’s a framework designed with simplicity in mind and offers a powerful and elegant API to build isolated tests, snapshot comparison, mocking, test coverage, and much more.

React Testing Library

React Testing Library is a JavaScript testing utility built specifically for testing React components. It simulates user interactions on isolated components and asserts their outputs to ensure the UI is behaving correctly.

#react #javascript #testing #jest

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How native is React Native? | React Native vs Native App Development

If you are undertaking a mobile app development for your start-up or enterprise, you are likely wondering whether to use React Native. As a popular development framework, React Native helps you to develop near-native mobile apps. However, you are probably also wondering how close you can get to a native app by using React Native. How native is React Native?

In the article, we discuss the similarities between native mobile development and development using React Native. We also touch upon where they differ and how to bridge the gaps. Read on.

A brief introduction to React Native

Let’s briefly set the context first. We will briefly touch upon what React Native is and how it differs from earlier hybrid frameworks.

React Native is a popular JavaScript framework that Facebook has created. You can use this open-source framework to code natively rendering Android and iOS mobile apps. You can use it to develop web apps too.

Facebook has developed React Native based on React, its JavaScript library. The first release of React Native came in March 2015. At the time of writing this article, the latest stable release of React Native is 0.62.0, and it was released in March 2020.

Although relatively new, React Native has acquired a high degree of popularity. The “Stack Overflow Developer Survey 2019” report identifies it as the 8th most loved framework. Facebook, Walmart, and Bloomberg are some of the top companies that use React Native.

The popularity of React Native comes from its advantages. Some of its advantages are as follows:

• Performance: It delivers optimal performance.
• Cross-platform development: You can develop both Android and iOS apps with it. The reuse of code expedites development and reduces costs.
• UI design: React Native enables you to design simple and responsive UI for your mobile app.
• 3rd party plugins: This framework supports 3rd party plugins.
• Developer community: A vibrant community of developers support React Native.

Why React Native is fundamentally different from earlier hybrid frameworks

Are you wondering whether React Native is just another of those hybrid frameworks like Ionic or Cordova? It’s not! React Native is fundamentally different from these earlier hybrid frameworks.

React Native is very close to native. Consider the following aspects as described on the React Native website:

• Access to many native platforms features: The primitives of React Native render to native platform UI. This means that your React Native app will use many native platform APIs as native apps would do.
• Near-native user experience: React Native provides several native components, and these are platform agnostic.
• The ease of accessing native APIs: React Native uses a declarative UI paradigm. This enables React Native to interact easily with native platform APIs since React Native wraps existing native code.

Due to these factors, React Native offers many more advantages compared to those earlier hybrid frameworks. We now review them.

#android app #frontend #ios app #mobile app development #benefits of react native #is react native good for mobile app development #native vs #pros and cons of react native #react mobile development #react native development #react native experience #react native framework #react native ios vs android #react native pros and cons #react native vs android #react native vs native #react native vs native performance #react vs native #why react native #why use react native

Interaction Testing with React Testing Library

Testing is complicated. I’ve certainly never been good at it. For the longest time, I’ve only been focused on basic function input-output unit tests. Why? Because they were easy — you didn’t need to render HTML, you didn’t need to query DOM elements, you didn’t need to interact with said DOM elements. But of course, React component testing is a necessity for any mature codebase. And it finally came time for me to sit down and figure it out.

That’s when I discovered React Testing Library. And suddenly, everything seemingly became much simpler. All the complexities that I’ve encountered, but not understood, that made me put off React component testing disappeared. Hopefully, the same will happen for you.

#react-testing-library #unit-testing #react #jest #interaction-testing

Simulate Browser Interactions with Testing Library’s UserEvent

My Journey

Like most, when I first started using Testing Library, I used Fire Event to test component interactions. After all, this API shipped with the library itself and was used in the test examples in the documentation. But I soon discovered that Fire Event had serious limitations. I would try clicking something and the expected effect did not happen. Why?

Browser Events

To understand this issue, we need to better understand browser events. When a user clicks something in their browser, multiple events are triggered — mouseDown, mouseUp, click, and focus. Similarly, when typing something, the keyDown, keyUp, and keyPress events all trigger! Because a single user interaction could trigger multiple events, developers have multiple options for implementation. This is where I ran into my issue.

Fire Event

Fire Event, unfortunately, requires you to use the method for the corresponding event handler to trigger. If an element has an onClick event handler, I have to use fireEvent.click; if an element has an onMouseDown event handler, I have to use fireEvent.mouseDown. In other words, I need to know the exact implementation of the event handler to successfully use fireEvent.

#react #jest #integration-testing #unit-testing #react-testing-library #react native

React Testing Tutorial (Jest + React Testing Library)

Learn how to write unit tests for your React components using Jest and the React Testing Library.

We will be writing some unit tests for React components using the Jest framework (https://jestjs.io/). We will begin by creating a react app using create-react-app. Then we will create a component & write some unit tests for it. We will finish by writing some snapshot tests to verify changes to the component tree.

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#react #jest #react testing library

Set Up Next.js Testing with Cypress, Jest, and React Testing Library

Learn how to set up Next.js with three commonly used testing tools: Cypress, Jest, and React Testing Library.

Cypress

Cypress is a test runner used for End-to-End (E2E) and Integration Testing.

Quickstart

You can use create-next-app with the with-cypress example to quickly get started.

npx create-next-app --example with-cypress with-cypress-app

Manual setup

To get started with Cypress, install the cypress package:

npm install --save-dev cypress

Add Cypress to the package.json scripts field:

"scripts": {
  "dev": "next dev",
  "build": "next build",
  "start": "next start",
  "cypress": "cypress open",
}

Run Cypress for the first time to generate examples that use their recommended folder structure:

npm run cypress

You can look through the generated examples and the Writing Your First Test section of the Cypress Documentation to help you get familiar with Cypress.

Creating your first Cypress integration test

Assuming the following two Next.js pages:

// pages/index.js
import Link from 'next/link'

export default function Home() {
  return (
    <nav>
      <Link href="/about">
        <a>About</a>
      </Link>
    </nav>
  )
}

// pages/about.js
export default function About() {
  return (
    <div>
      <h1>About Page</h1>
    </div>
  )
}

Add a test to check your navigation is working correctly:

// cypress/integration/app.spec.js

describe('Navigation', () => {
  it('should navigate to the about page', () => {
    // Start from the index page
    cy.visit('http://localhost:3000/')

    // Find a link with an href attribute containing "about" and click it
    cy.get('a[href*="about"]').click()

    // The new url should include "/about"
    cy.url().should('include', '/about')

    // The new page should contain an h1 with "About page"
    cy.get('h1').contains('About Page')
  })
})

You can use cy.visit("/") instead of cy.visit("http://localhost:3000/") if you add "baseUrl": "http://localhost:3000" to the cypress.json configuration file.

Running your Cypress tests

Since Cypress is testing a real Next.js application, it requires the Next.js server to be running prior to starting Cypress. We recommend running your tests against your production code to more closely resemble how your application will behave.

Run npm run build and npm run start, then run npm run cypress in another terminal window to start Cypress.

Note: Alternatively, you can install the start-server-and-test package and add it to the package.json scripts field: "test": "start-server-and-test start http://localhost:3000 cypress" to start the Next.js production server in conjuction with Cypress. Remember to rebuild your application after new changes.

Getting ready for Continuous Integration (CI)

You will have noticed that running Cypress so far has opened an interactive browser which is not ideal for CI environments. You can also run Cypress headlessly using the cypress run command:

// package.json

"scripts": {
  //...
  "cypress": "cypress open",
  "cypress:headless": "cypress run",
  "e2e": "start-server-and-test start http://localhost:3000 cypress",
  "e2e:headless": "start-server-and-test start http://localhost:3000 cypress:headless"
}

You can learn more about Cypress and Continuous Integration from these resources:

• Cypress Continuous Integration Docs
• Cypress GitHub Actions Guide
• Official Cypress Github Action

Jest and React Testing Library

Jest and React Testing Library are frequently used together for Unit Testing.

Quickstart

You can use create-next-app with the with-jest example to quickly get started with Jest and React Testing Library:

npx create-next-app --example with-jest with-jest-app

Manual setup

To manually set up Jest and React Testing Library, install jest , @testing-library/react, @testing-library/jest-dom as well as some supporting packages:

npm install --save-dev jest babel-jest @testing-library/react @testing-library/jest-dom identity-obj-proxy react-test-renderer

Configuring Jest

Create a jest.config.js file in your project's root directory and add the following configuration options:

// jest.config.js

module.exports = {
  collectCoverageFrom: [
    '**/*.{js,jsx,ts,tsx}',
    '!**/*.d.ts',
    '!**/node_modules/**',
  ],
  moduleNameMapper: {
    /* Handle CSS imports (with CSS modules)
    https://jestjs.io/docs/webpack#mocking-css-modules */
    '^.+\\.module\\.(css|sass|scss)$': 'identity-obj-proxy',

    // Handle CSS imports (without CSS modules)
    '^.+\\.(css|sass|scss)$': '<rootDir>/__mocks__/styleMock.js',

    /* Handle image imports
    https://jestjs.io/docs/webpack#handling-static-assets */
    '^.+\\.(jpg|jpeg|png|gif|webp|svg)$': '<rootDir>/__mocks__/fileMock.js',
  },
  testPathIgnorePatterns: ['<rootDir>/node_modules/', '<rootDir>/.next/'],
  testEnvironment: 'jsdom',
  transform: {
    /* Use babel-jest to transpile tests with the next/babel preset
    https://jestjs.io/docs/configuration#transform-objectstring-pathtotransformer--pathtotransformer-object */
    '^.+\\.(js|jsx|ts|tsx)$': ['babel-jest', { presets: ['next/babel'] }],
  },
  transformIgnorePatterns: [
    '/node_modules/',
    '^.+\\.module\\.(css|sass|scss)$',
  ],
}

You can learn more about each option above in the Jest docs.

Handling stylesheets and image imports

These files aren't useful in tests but importing them may cause errors, so we will need to mock them. Create the mock files we referenced in the configuration above - fileMock.js and styleMock.js - inside a __mocks__ directory:

// __mocks__/fileMock.js

(module.exports = "test-file-stub")

// __mocks__/styleMock.js

module.exports = {};

For more information on handling static assets, please refer to the Jest Docs.

Extend Jest with custom matchers

@testing-library/jest-dom includes a set of convenient custom matchers such as .toBeInTheDocument() making it easier to write tests. You can import the custom matchers for every test by adding the following option to the Jest configuration file:

// jest.config.js

setupFilesAfterEnv: ['<rootDir>/jest.setup.js']

Then, inside jest.setup.js, add the following import:

// jest.setup.js

import '@testing-library/jest-dom/extend-expect'

If you need to add more setup options before each test, it's common to add them to the jest.setup.js file above.

Absolute Imports and Module Path Aliases

If your project is using Module Path Aliases, you will need to configure Jest to resolve the imports by matching the paths option in the jsconfig.json file with the moduleNameMapper option in the jest.config.js file. For example:

// tsconfig.json or jsconfig.json
{
  "compilerOptions": {
    "baseUrl": ".",
    "paths": {
      "@/components/*": ["components/*"]
    }
  }
}

// jest.config.js
moduleNameMapper: {
  '^@/components/(.*)$': '<rootDir>/components/$1',
}

Add a test script to package.json

Add the Jest executable in watch mode to the package.json scripts:

"scripts": {
  "dev": "next dev",
  "build": "next build",
  "start": "next start",
  "test": "jest --watch"
}

jest --watch will re-run tests when a file is changed. For more Jest CLI options, please refer to the Jest Docs.

Create your first tests

Your project is now ready to run tests. Follow Jests convention by adding tests to the __tests__ folder in your project's root directory.

For example, we can add a test to check if the <Index /> component successfully renders a heading:

// __tests__/index.test.jsx

/**
 * @jest-environment jsdom
 */

import React from 'react'
import { render, screen } from '@testing-library/react'
import Home from '../pages/index'

describe('Home', () => {
  it('renders a heading', () => {
    render(<Home />)

    const heading = screen.getByRole('heading', {
      name: /welcome to next\.js!/i,
    })

    expect(heading).toBeInTheDocument()
  })
})

Note: The @jest-environment jsdom comment above configures the testing environment as jsdom inside the test file because React Testing Library uses DOM elements like document.body which will not work in Jest's default node testing environment. Alternatively, you can also set the jsdom environment globally by adding the Jest configuration option: "testEnvironment": "jsdom" in jest.config.js.

Optionally, add a snapshot test to keep track of any unexpected changes to your <Index /> component:

// __tests__/snapshot.js
import React from 'react'
import renderer from 'react-test-renderer'
import Index from '../pages/index'

it('renders homepage unchanged', () => {
  const tree = renderer.create(<Index />).toJSON()
  expect(tree).toMatchSnapshot()
})

Note: Test files should not be included inside the pages directory because any files inside the pages directory are considered routes.

Running your test suite

Run npm run jest to run your test suite. After your tests pass or fail, you will notice a list of interactive Jest commands that will be helpful as you add more tests.

#next #testing #cypress #jest #react #test #webdev