Gain insights into writing automated tests for React apps with Jest and React Testing Library. Learn about simulating user interactions and testing asynchronous parts to enhance app reliability.

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The skills acquired from this course will allow you to write automated tests on your React app using Jest and React Testing Library, which are the de-facto testing tools for React. These tests will allow you to ship new versions of your app with confidence. The tests will also allow you to increase your release frequency, allowing you to get features to your customers quicker.

You will learn how to write robust tests that are resilient to breaking when code is improved and refactored, reducing maintenance costs.
The course will teach you how to simulate a user interacting with the app. Beyond that, it will also teach you how to robustly test asynchronous parts of the app.

Regular quizzes and exercises will reinforce knowledge throughout the course.

Testing React Apps with Jest and React Testing Library

# Creating our first component test

The [starter project](https://github.com/carlrip/testing-react-apps-with-jest-and-rtl-code/tree/master/Chapter-04/Lesson-01/start) for this lesson was created by [Create React App](https://create-react-app.dev/). Jest is already installed and configured and is ready for us to write a test.

There is a copy of the starter project that you can use in this lesson  below:

# Getting Started with Create React App

This project was bootstrapped with [Create React App](https://github.com/facebook/create-react-app).

## Available Scripts

In the project directory, you can run:

### `npm start`

Runs the app in the development mode.\
Open [http://localhost:3000](http://localhost:3000) to view it in the browser.

The page will reload if you make edits.\
You will also see any lint errors in the console.

### `npm test`

Launches the test runner in the interactive watch mode.\
See the section about [running tests](https://facebook.github.io/create-react-app/docs/running-tests) for more information.

### `npm run build`

Builds the app for production to the `build` folder.\
It correctly bundles React in production mode and optimizes the build for the best performance.

The build is minified and the filenames include the hashes.\
Your app is ready to be deployed!

See the section about [deployment](https://facebook.github.io/create-react-app/docs/deployment) for more information.

### `npm run eject`

**Note: this is a one-way operation. Once you `eject`, you can’t go back!**

If you aren’t satisfied with the build tool and configuration choices, you can `eject` at any time. This command will remove the single build dependency from your project.

Instead, it will copy all the configuration files and the transitive dependencies (webpack, Babel, ESLint, etc) right into your project so you have full control over them. All of the commands except `eject` will still work, but they will point to the copied scripts so you can tweak them. At this point you’re on your own.

You don’t have to ever use `eject`. The curated feature set is suitable for small and middle deployments, and you shouldn’t feel obligated to use this feature. However we understand that this tool wouldn’t be useful if you couldn’t customize it when you are ready for it.

## Learn More

You can learn more in the [Create React App documentation](https://facebook.github.io/create-react-app/docs/getting-started).

To learn React, check out the [React documentation](https://reactjs.org/).

### Code Splitting

This section has moved here: [https://facebook.github.io/create-react-app/docs/code-splitting](https://facebook.github.io/create-react-app/docs/code-splitting)

### Analyzing the Bundle Size

This section has moved here: [https://facebook.github.io/create-react-app/docs/analyzing-the-bundle-size](https://facebook.github.io/create-react-app/docs/analyzing-the-bundle-size)

### Making a Progressive Web App

This section has moved here: [https://facebook.github.io/create-react-app/docs/making-a-progressive-web-app](https://facebook.github.io/create-react-app/docs/making-a-progressive-web-app)

### Advanced Configuration

This section has moved here: [https://facebook.github.io/create-react-app/docs/advanced-configuration](https://facebook.github.io/create-react-app/docs/advanced-configuration)

### Deployment

This section has moved here: [https://facebook.github.io/create-react-app/docs/deployment](https://facebook.github.io/create-react-app/docs/deployment)

### `npm run build` fails to minify

This section has moved here: [https://facebook.github.io/create-react-app/docs/troubleshooting#npm-run-build-fails-to-minify](https://facebook.github.io/create-react-app/docs/troubleshooting#npm-run-build-fails-to-minify)


import React from 'react';
import { shallow, configure } from 'enzyme';
import Adapter from 'enzyme-adapter-react-16';

configure({ adapter: new Adapter() });

import HelloWorld from './app';

var TestResult = function() {
    this.succeeded = false;
    this.reason = "";
    this.input = "";
    this.expected_output = "";
    this.actual_output = "";
}

export const executeTests = function() {

  var results = [];

  result = new TestResult();
  result.input = 'HelloWorld Component';
  result.expected_output = "span containing text 'Hello World'"

  let wrapper = shallow(<HelloWorld />);

  // Call your Challenge function here.

  let type = wrapper.type();
  let testSuccessful = true;
  let failureReason;

  if (type !== 'span') {
    testSuccessful = false;
    failureReason = "You need to render exactly one span HTML element";
  } else if (wrapper.props().children != "Hello World") {
     testSuccessful = false;
     failureReason = "You have rendered wrong message in your span element";
  }

  result.actual_output = wrapper.html();

  if (testSuccessful) {
    result.succeeded = true;
    result.reason = "Succeeded"
  } else {
    result.succeeded = false;
    result.reason = failureReason;
  }

  results.push(result);

  return results;
}


React

Dart

GoJS React

Typescript React

Vue.js

Open up the `src` folder in the starter project. We'll write a test for an `ErrorMessage` component contained within the folder. The test will check that `ErrorMessage` renders the correct text for a given `message` passed into it.

Let's start by creating a test file in the `src/ErrorMessage` folder called `ErrorMessage.test.js`. If you are using the code widget above, this file has already been added for you.

Add the following import statements to the test file:

```javascript
import { render } from "react-dom";
import { act } from "react-dom/test-utils";
import { ErrorMessage } from "./ErrorMessage";
```

We have imported the `ErrorMessage` component we are testing and a `render` function to render it.

We have also imported an `act` function. We will use this to ensure all the updates have been made to the component before carrying out checks on it.

Let’s write the test:

```javascript
test("Should render correct message when message prop passed", () => {});
```

Create a container element to place the `ErrorMessage` component in.

```javascript
test("Should render correct message when message prop passed", () => {
  const container = document.createElement("div");
  document.body.appendChild(container);
});
```

Render the `ErrorMessage` within the container element:

```javascript
test("Should render correct message when message prop passed", () => {
  const container = document.createElement("div");
  document.body.appendChild(container);

  act(() => {
    render(<ErrorMessage message="test" />, container);
  });
});
```

Retrieve a reference to the `div` element within the `ErrorMessage` component.

```javascript
test("Should render correct message when message prop passed", () => {
  const container = document.createElement("div");
  document.body.appendChild(container);

  act(() => {
    render(<ErrorMessage message="test" />, container);
  });

  const div = document.querySelector(".saveerror");
});
```

Lastly, check that the text content of the `div` element is as we expect. Add the final line of the test to check that `div.textContent` is `"test"`.

Learn how to create a component test without React Testing Library in this lesson.

Writing a Component Test without React Testing Library

Introduction

Getting Started with Jest

Using Matchers

Getting Started with React Testing Library

React Testing Library Queries

Firing Events

Mocking

Testing Different Types of Components

Appendix

Writing a Component Test without React Testing Library

Creating our first component test