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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Jest - lesson 2

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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

# Refactoring the component test

The starter project for this lesson is the project we finished in the last lesson. It contains a test on an `ErrorMessage` component that verifies that the value of the `message` prop is rendered correctly.

The starter project was created by [Create React App](https://create-react-app.dev/). So, along with Jest, React Testing Library is already installed and configured.

The starter code is available in the code widget 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

Let’s refactor the test in `ErrorMessage.test.js`. We'll start by removing the import statements for the `render` and `act` functions and replacing them with the following import statement:

```javascript
import { render, screen } from "@testing-library/react";
```

We’ll use the [`render`](https://testing-library.com/docs/angular-testing-library/api/#render) function from React Testing Library rather than the one in React. This will simplify the code we write to render the component. We'll learn more about `render` later in this course.

The `screen` object contains useful methods to select the element we want to check.

Now we can refactor the test. Remove the lines that create and add the container element to the DOM. We can also remove the line that renders the component and the `act` function call that wraps it. These lines can be replaced with the following line:

```jsx
render(<ErrorMessage message="test" />);
```

Under the hood, React Testing Library renders the component to the DOM inside a container element wrapped in an `act` function call. Essentially, it does the work for us so that we don’t need to manually repeat actions in every component test.

Next, we can replace the line that selects the element we want to check with the following line:

```javascript
const div = screen.getByText("test");
```

React Testing Library’s [`getByText`](https://testing-library.com/docs/queries/bytext) function will search through the DOM to find an element that contains the text `"test"`. The `getByText` function will raise an error if the element isn’t found.

The `getByText` function is one of React Testing Library’s **query** functions. We'll learn more about `getByText` and other queries later in this course.

React Testing Library automatically removes test elements it creates for us, so the line that removes the `div` at the end of the test can be removed.

The refactored test should now read as follows:

```jsx
test("Should render correct message when message prop passed", () => {
  render(<ErrorMessage message="test" />);
  const div = screen.getByText("test");
  expect(div.textContent).toBe("test");
});
```

If we run the test, it will pass.

Although the test passes, it can be improved. If we think about it, the expectation is a bit redundant:

```javascript
expect(div.textContent).toBe("test");
```

The `div` element’s text content must be `"test"`, or an error will be raised in the `getByText` call. A better check would be to check that the element is in the document. We can do this using the `toBeInTheDocument` matcher as follows:

```javascript
expect(div).toBeInTheDocument();
```

It is worth noting that `toBeInTheDocument` isn't a matcher available in the core Jest library. Instead, this is available in a supporting library called `jest-dom`, which was installed and configured by Create React App.

The test will still pass after we make these changes.

We can simplify the test even more by adding the query to the expectation and removing the `div` variable assignment:

```javascript
expect(screen.getByText("test")).toBeInTheDocument();
```

The test will pass again after this change.

The full test code is as follows:

```jsx
test("Should render correct message when message prop passed", () => {
  render(<ErrorMessage message="test" />);
  expect(screen.getByText("React Testing Libraries test")).toBeInTheDocument();
});
```

This is much simpler!

# Refactoring the component

We are now going to refactor the component as we did in the last lesson.

We will change the CSS class name to `error-message` so that the name is a bit more generic.

Change the class name in the `ErrorMessage.css` file from `save-error` to `error-message`:

```css
.error-message {
  ...;
}
.error-message img {
  ...;
}
```

Change the referenced CSS class name in the `ErrorMessage.js` file:

```jsx
<div className="error-message">
```

The tests should still pass.

We didn’t change the component’s functionality or behavior—instead, we made a little improvement to the code. This is precisely what we should aim for.

To check that the element we wanted was selected, we used `getByText`. This function doesn’t reference any implementation details such as a CSS class name, a DOM id, or DOM structure, so if these implementation details change, they won’t break the test.

Although `getByText` references the text in the DOM, it is not an implementation detail. Rather, it is a detail that the user sees on the page and not a technical detail. This detail should only change when there is a change to the requirements of the component.


Learn how to refactor the component test that was implemented in the last lesson to produce robust tests using 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 with React Testing Library

Refactoring the component test