---
id: tutorial-react
title: Testing React Apps
---

At Facebook, we use Jest to test [React](https://reactjs.org/) applications.

## Setup

### Setup with Create React App

If you are new to React, we recommend using [Create React App](https://create-react-app.dev/). It is ready to use and [ships with Jest](https://create-react-app.dev/docs/running-tests/#docsNav)! You will only need to add `react-test-renderer` for rendering snapshots.

Run

```bash npm2yarn
npm install --save-dev react-test-renderer
```

### Setup without Create React App

If you have an existing application you'll need to install a few packages to make everything work well together. We are using the `babel-jest` package and the `react` babel preset to transform our code inside of the test environment. Also see [using babel](GettingStarted.md#using-babel).

Run

```bash npm2yarn
npm install --save-dev jest babel-jest @babel/preset-env @babel/preset-react react-test-renderer
```

Your `package.json` should look something like this (where `<current-version>` is the actual latest version number for the package). Please add the scripts and jest configuration entries:

```json
{
  "dependencies": {
    "react": "<current-version>",
    "react-dom": "<current-version>"
  },
  "devDependencies": {
    "@babel/preset-env": "<current-version>",
    "@babel/preset-react": "<current-version>",
    "babel-jest": "<current-version>",
    "jest": "<current-version>",
    "react-test-renderer": "<current-version>"
  },
  "scripts": {
    "test": "jest"
  }
}
```

```js title="babel.config.js"
module.exports = {
  presets: [
    '@babel/preset-env',
    ['@babel/preset-react', {runtime: 'automatic'}],
  ],
};
```

**And you're good to go!**

### Snapshot Testing

Let's create a [snapshot test](SnapshotTesting.md) for a Link component that renders hyperlinks:

```tsx title="Link.js"
import {useState} from 'react';

const STATUS = {
  HOVERED: 'hovered',
  NORMAL: 'normal',
};

export default function Link({page, children}) {
  const [status, setStatus] = useState(STATUS.NORMAL);

  const onMouseEnter = () => {
    setStatus(STATUS.HOVERED);
  };

  const onMouseLeave = () => {
    setStatus(STATUS.NORMAL);
  };

  return (
    <a
      className={status}
      href={page || '#'}
      onMouseEnter={onMouseEnter}
      onMouseLeave={onMouseLeave}
    >
      {children}
    </a>
  );
}
```

:::note

Examples are using Function components, but Class components can be tested in the same way. See [React: Function and Class Components](https://reactjs.org/docs/components-and-props.html#function-and-class-components). **Reminders** that with Class components, we expect Jest to be used to test props and not methods directly.

:::

Now let's use React's test renderer and Jest's snapshot feature to interact with the component and capture the rendered output and create a snapshot file:

```tsx title="Link.test.js"
import renderer from 'react-test-renderer';
import Link from '../Link';

it('changes the class when hovered', () => {
  const component = renderer.create(
    <Link page="http://www.facebook.com">Facebook</Link>,
  );
  let tree = component.toJSON();
  expect(tree).toMatchSnapshot();

  // manually trigger the callback
  renderer.act(() => {
    tree.props.onMouseEnter();
  });
  // re-rendering
  tree = component.toJSON();
  expect(tree).toMatchSnapshot();

  // manually trigger the callback
  renderer.act(() => {
    tree.props.onMouseLeave();
  });
  // re-rendering
  tree = component.toJSON();
  expect(tree).toMatchSnapshot();
});
```

When you run `yarn test` or `jest`, this will produce an output file like this:

```javascript title="__tests__/__snapshots__/Link.test.js.snap"
exports[`changes the class when hovered 1`] = `
<a
  className="normal"
  href="http://www.facebook.com"
  onMouseEnter={[Function]}
  onMouseLeave={[Function]}
>
  Facebook
</a>
`;

exports[`changes the class when hovered 2`] = `
<a
  className="hovered"
  href="http://www.facebook.com"
  onMouseEnter={[Function]}
  onMouseLeave={[Function]}
>
  Facebook
</a>
`;

exports[`changes the class when hovered 3`] = `
<a
  className="normal"
  href="http://www.facebook.com"
  onMouseEnter={[Function]}
  onMouseLeave={[Function]}
>
  Facebook
</a>
`;
```

The next time you run the tests, the rendered output will be compared to the previously created snapshot. The snapshot should be committed along with code changes. When a snapshot test fails, you need to inspect whether it is an intended or unintended change. If the change is expected you can invoke Jest with `jest -u` to overwrite the existing snapshot.

The code for this example is available at [examples/snapshot](https://github.com/facebook/jest/tree/main/examples/snapshot).

#### Snapshot Testing with Mocks, Enzyme and React 16+

There's a caveat around snapshot testing when using Enzyme and React 16+. If you mock out a module using the following style:

```js
jest.mock('../SomeDirectory/SomeComponent', () => 'SomeComponent');
```

Then you will see warnings in the console:

```bash
Warning: <SomeComponent /> is using uppercase HTML. Always use lowercase HTML tags in React.

# Or:
Warning: The tag <SomeComponent> is unrecognized in this browser. If you meant to render a React component, start its name with an uppercase letter.
```

React 16 triggers these warnings due to how it checks element types, and the mocked module fails these checks. Your options are:

1.  Render as text. This way you won't see the props passed to the mock component in the snapshot, but it's straightforward:
    ```js
    jest.mock('./SomeComponent', () => () => 'SomeComponent');
    ```
2.  Render as a custom element. DOM "custom elements" aren't checked for anything and shouldn't fire warnings. They are lowercase and have a dash in the name.
    ```tsx
    jest.mock('./Widget', () => () => <mock-widget />);
    ```
3.  Use `react-test-renderer`. The test renderer doesn't care about element types and will happily accept e.g. `SomeComponent`. You could check snapshots using the test renderer, and check component behavior separately using Enzyme.
4.  Disable warnings all together (should be done in your jest setup file):
    ```js
    jest.mock('fbjs/lib/warning', () => require('fbjs/lib/emptyFunction'));
    ```
    This shouldn't normally be your option of choice as useful warnings could be lost. However, in some cases, for example when testing react-native's components we are rendering react-native tags into the DOM and many warnings are irrelevant. Another option is to swizzle the console.warn and suppress specific warnings.

### DOM Testing

If you'd like to assert, and manipulate your rendered components you can use [@testing-library/react](https://github.com/testing-library/react-testing-library), [Enzyme](https://enzymejs.github.io/enzyme/), or React's [TestUtils](https://reactjs.org/docs/test-utils.html). The following example use `@testing-library/react`.

#### @testing-library/react

```bash npm2yarn
npm install --save-dev @testing-library/react
```

Let's implement a checkbox which swaps between two labels:

```tsx title="CheckboxWithLabel.js"
import {useState} from 'react';

export default function CheckboxWithLabel({labelOn, labelOff}) {
  const [isChecked, setIsChecked] = useState(false);

  const onChange = () => {
    setIsChecked(!isChecked);
  };

  return (
    <label>
      <input type="checkbox" checked={isChecked} onChange={onChange} />
      {isChecked ? labelOn : labelOff}
    </label>
  );
}
```

```tsx title="__tests__/CheckboxWithLabel-test.js"
import {cleanup, fireEvent, render} from '@testing-library/react';
import CheckboxWithLabel from '../CheckboxWithLabel';

// Note: running cleanup afterEach is done automatically for you in @testing-library/react@9.0.0 or higher
// unmount and cleanup DOM after the test is finished.
afterEach(cleanup);

it('CheckboxWithLabel changes the text after click', () => {
  const {queryByLabelText, getByLabelText} = render(
    <CheckboxWithLabel labelOn="On" labelOff="Off" />,
  );

  expect(queryByLabelText(/off/i)).toBeTruthy();

  fireEvent.click(getByLabelText(/off/i));

  expect(queryByLabelText(/on/i)).toBeTruthy();
});
```

The code for this example is available at [examples/react-testing-library](https://github.com/facebook/jest/tree/main/examples/react-testing-library).

### Custom transformers

If you need more advanced functionality, you can also build your own transformer. Instead of using `babel-jest`, here is an example of using `@babel/core`:

```javascript title="custom-transformer.js"
'use strict';

const {transform} = require('@babel/core');
const jestPreset = require('babel-preset-jest');

module.exports = {
  process(src, filename) {
    const result = transform(src, {
      filename,
      presets: [jestPreset],
    });

    return result || src;
  },
};
```

Don't forget to install the `@babel/core` and `babel-preset-jest` packages for this example to work.

To make this work with Jest you need to update your Jest configuration with this: `"transform": {"\\.js$": "path/to/custom-transformer.js"}`.

If you'd like to build a transformer with babel support, you can also use `babel-jest` to compose one and pass in your custom configuration options:

```javascript
const babelJest = require('babel-jest');

module.exports = babelJest.createTransformer({
  presets: ['my-custom-preset'],
});
```

See [dedicated docs](CodeTransformation.md#writing-custom-transformers) for more details.
