Testing React Native Apps
At Facebook, we use Jest to test React Native applications.
Get a deeper insight into testing a working React Native app example by reading the following series: Part 1: Jest – Snapshot come into play and Part 2: Jest – Redux Snapshots for your Actions and Reducers.
Setup
Starting from react-native version 0.38, a Jest setup is included by default when running react-native init
. The following configuration should be automatically added to your package.json file:
{
"scripts": {
"test": "jest"
},
"jest": {
"preset": "react-native"
}
}
Run yarn test
to run tests with Jest.
If you are upgrading your react-native application and previously used the jest-react-native
preset, remove the dependency from your package.json
file and change the preset to react-native
instead.
Snapshot Test
Let's create a snapshot test for a small intro component with a few views and text components and some styles:
import React, {Component} from 'react';
import {StyleSheet, Text, View} from 'react-native';
class Intro extends Component {
render() {
return (
<View style={styles.container}>
<Text style={styles.welcome}>Welcome to React Native!</Text>
<Text style={styles.instructions}>
This is a React Native snapshot test.
</Text>
</View>
);
}
}
const styles = StyleSheet.create({
container: {
alignItems: 'center',
backgroundColor: '#F5FCFF',
flex: 1,
justifyContent: 'center',
},
instructions: {
color: '#333333',
marginBottom: 5,
textAlign: 'center',
},
welcome: {
fontSize: 20,
margin: 10,
textAlign: 'center',
},
});
export default Intro;
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:
import React from 'react';
import renderer from 'react-test-renderer';
import Intro from '../Intro';
test('renders correctly', () => {
const tree = renderer.create(<Intro />).toJSON();
expect(tree).toMatchSnapshot();
});
When you run yarn test
or jest
, this will produce an output file like this:
exports[`Intro renders correctly 1`] = `
<View
style={
Object {
"alignItems": "center",
"backgroundColor": "#F5FCFF",
"flex": 1,
"justifyContent": "center",
}
}>
<Text
style={
Object {
"fontSize": 20,
"margin": 10,
"textAlign": "center",
}
}>
Welcome to React Native!
</Text>
<Text
style={
Object {
"color": "#333333",
"marginBottom": 5,
"textAlign": "center",
}
}>
This is a React Native snapshot test.
</Text>
</View>
`;
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/react-native.
Preset configuration
The preset sets up the environment and is very opinionated and based on what we found to be useful at Facebook. All of the configuration options can be overwritten just as they can be customized when no preset is used.
Environment
react-native
ships with a Jest preset, so the jest.preset
field of your package.json
should point to react-native
. The preset is a node environment that mimics the environment of a React Native app. Because it doesn't load any DOM or browser APIs, it greatly improves Jest's startup time.
transformIgnorePatterns customization
The transformIgnorePatterns
option can be used to specify which files shall be transformed by Babel. Many react-native
npm modules unfortunately don't pre-compile their source code before publishing.
By default the jest-react-native
preset only processes the project's own source files and react-native
. If you have npm dependencies that have to be transformed you can customize this configuration option by including modules other than react-native
by grouping them and separating them with the |
operator:
{
"transformIgnorePatterns": [
"node_modules/(?!(react-native|my-project|react-native-button)/)"
]
}
You can test which paths would match (and thus be excluded from transformation) with a tool like this.
transformIgnorePatterns
will exclude a file from transformation if the path matches against any pattern provided. Splitting into multiple patterns could therefore have unintended results if you are not careful. In the example below, the exclusion (also known as a negative lookahead assertion) for foo
and bar
cancel each other out:
{
"transformIgnorePatterns": ["node_modules/(?!foo/)", "node_modules/(?!bar/)"] // not what you want
}
setupFiles
If you'd like to provide additional configuration for every test file, the setupFiles
configuration option can be used to specify setup scripts.
moduleNameMapper
The moduleNameMapper
can be used to map a module path to a different module. By default the preset maps all images to an image stub module but if a module cannot be found this configuration option can help:
{
"moduleNameMapper": {
"my-module.js": "<rootDir>/path/to/my-module.js"
}
}