Testing React Apps

At Facebook, we use Jest to test React applications.

Setup #

If you are just getting started with React, we recommend using create-react-app. It is ready to use and ships with Jest!

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.

Run

npm install --save-dev jest babel-jest babel-preset-es2015 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:

// package.json
  "dependencies": {
    "react": "<current-version>",
    "react-dom": "<current-version>"
  },
  "devDependencies": {
    "babel-jest": "<current-version>",
    "babel-preset-es2015": "<current-version>",
    "babel-preset-react": "<current-version>",
    "jest": "<current-version>",
    "react-test-renderer": "<current-version>"
  },
  "scripts": {
    "test": "jest"
  }

// .babelrc
{
  "presets": ["es2015", "react"]
}

And you're good to go!

Snapshot Testing #

Let's create a snapshot test for a Link component that renders hyperlinks:

// Link.react.js
import React from 'react';

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

export default class Link extends React.Component {

  constructor(props) {
    super(props);

    this._onMouseEnter = this._onMouseEnter.bind(this);
    this._onMouseLeave = this._onMouseLeave.bind(this);

    this.state = {
      class: STATUS.NORMAL,
    };
  }

  _onMouseEnter() {
    this.setState({class: STATUS.HOVERED});
  }

  _onMouseLeave() {
    this.setState({class: STATUS.NORMAL});
  }

  render() {
    return (
      <a
        className={this.state.class}
        href={this.props.page || '#'}
        onMouseEnter={this._onMouseEnter}
        onMouseLeave={this._onMouseLeave}>
        {this.props.children}
      </a>
    );
  }

}

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:

// Link.react-test.js
import React from 'react';
import Link from '../Link.react';
import renderer from 'react-test-renderer';

test('Link 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
  tree.props.onMouseEnter();
  // re-rendering
  tree = component.toJSON();
  expect(tree).toMatchSnapshot();

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

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

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

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

exports[`Link 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 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.

DOM Testing #

If you'd like to assert, and manipulate your rendered components you can use Enzyme or React's TestUtils. We use Enzyme for this example.

You have to run npm install --save-dev enzyme react-addons-test-utils to use Enzyme (react-addons-test-utils is a peer dependency).

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

// CheckboxWithLabel.js

import React from 'react';

export default class CheckboxWithLabel extends React.Component {

  constructor(props) {
    super(props);
    this.state = {isChecked: false};

    // bind manually because React class components don't auto-bind
    // http://facebook.github.io/react/blog/2015/01/27/react-v0.13.0-beta-1.html#autobinding
    this.onChange = this.onChange.bind(this);
  }

  onChange() {
    this.setState({isChecked: !this.state.isChecked});
  }

  render() {
    return (
      <label>
        <input
          type="checkbox"
          checked={this.state.isChecked}
          onChange={this.onChange}
        />
        {this.state.isChecked ? this.props.labelOn : this.props.labelOff}
      </label>
    );
  }
}

We use Enzyme's shallow renderer in this example.

// __tests__/CheckboxWithLabel-test.js

import React from 'react';
import {shallow} from 'enzyme';
import CheckboxWithLabel from '../CheckboxWithLabel';

test('CheckboxWithLabel changes the text after click', () => {
  // Render a checkbox with label in the document
  const checkbox = shallow(
    <CheckboxWithLabel labelOn="On" labelOff="Off" />
  );

  expect(checkbox.text()).toEqual('Off');

  checkbox.find('input').simulate('change');

  expect(checkbox.text()).toEqual('On');
});

The code for this example is available at examples/enzyme.

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:

// custom-transformer.js
'use strict';

const babel = require('babel-core');
const jestPreset = require('babel-preset-jest');

module.exports = {
  process(src, filename) {
    if (babel.util.canCompile(filename)) {
      return babel.transform(src, {
        filename,
        presets: [jestPreset],
        retainLines: true,
      }).code;
    }
    return 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:

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

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