Adding Jest Snapshot Testing to React Native Windows Apps

Chiara Mooney

React Native Gallery is a React Native Windows app which showcases core and community module component samples. One function of the app is to validate whether a release has caused UI changes to our component set. In the past, this process was done manually by upgrading the app and walking through all sample pages to confirm that components were being rendered as expected, following the version upgrade. This process was time consuming and lacked accuracy. To automate more of this process, we’ve begun to add UI testing to React Native Gallery. In this blogpost, I’ll walk through how I added Jest Snapshot testing to React Native Gallery. While this guide is specifically about adding Jest testing to a React Native Windows app, the content is also applicable to cross-platform apps as well.

One of the most common testing frameworks to use with React Native apps is Jest. Jest is a JavaScript testing framework that is included by default within React Native apps that were created after v0.38. Jest tests can be shared across all of the platforms that your app targets. You can confirm that your app has Jest setup by checking the package.json file at the root of your app. It should contain the following code:

  "scripts": {
    "test": "jest"
  "jest": {
    "preset": "react-native"

Snapshot tests help to capture UI and verify that no unexpected UI changes have been made. A snapshot test case will include rendering a UI component, taking a snapshot, and then comparing the snapshot to a reference file. If the two do not match, the test will fail. This will occur in one of two cases: either the UI change is unexpected, or the change is expected – and the snapshot reference needs to be updated.

To begin, I followed Jest’s Getting Started guide for adding testing to React Native apps. This gave me a good overview of potential Jest configurations/customizations, how to write a simple snapshot test, and information on snapshot reference files.

Writing snapshot tests in Jest is fairly simple. Once you have your App-test.js file, you’ll want to import the desired component that you want to snapshot and write a test case. Here is an example snippet from React Native Gallery’s testing of its Button component page:

import {ButtonExamplePage} from '../src/examples/ButtonExamplePage';

test('Button Example Page', () => {
  const tree = create(<ButtonExamplePage />).toJSON();

As I began to work on this, I realized there was some additional configuration I needed to add in order to get my tests to run correctly. First, I had several modules that needed to be added to the transformIgnorePatterns Jest customization. This customization is used to specify which files should be transformed by Babel. By default, Jest only processes the app’s and react-native’s source code. You’ll know if a module needs to be transformed, if the app throws an error saying “Jest encountered an unexpected token”. In this case, the module should be listed in the transformIgnorePatterns customization within your app’s package.json. See Jest’s transformIgnorePatterns documentation for syntax info.

Next, I had several modules which required mocking. Mock functions are often needed for modules which implement API’s instead of UI components. Examples within React Native Gallery include the react-native-device-info module or the react-navigation module. If a function needs to be mocked by Jest, you’ll commonly receive an error that the method is undefined, when trying to render a component which makes use of the method. Here’s an example error for a react-native-device-info method that needs mocking: “@react-native-community/react-native-device-info: NativeModule.RNDeviceInfo is null”. To add mocks to your Jest configuration, you’ll want to create a jest-setup.js file at the root of your repo. Then you’ll want to edit your Jest configuration in your app’s package.json to identify that you’ve created a setup file (see the package.json source snippet below for syntax). Now you’ll want to add contents to your jest-setup.js file. Some community modules already have mock.js files implemented in their source. If this is the case, you’ll just want to reference this file in your setup file.

Here is an example of the code added for the react-native-permissions module:

jest.mock('react-native-permissions', () =>

If the community module does not have a mock.js file, you’ll have to mock the undefined function yourself. There are a couple ways to sleuth out how to correctly mock:

  1. Check the community module source. If they have an example app with Jest testing, what do they do in their jest-setup.js file?
  2. You can also check forum posts from the community on how they mocked particular methods for a given module.
  3. React Native Gallery has a wide set of community modules it has tests for. Check out our jest-setup.js to see how we mocked needed modules.

Here is React Native Gallery’s Jest customization within package.json:

"jest": {
    "preset": "react-native",
    "modulePathIgnorePatterns": [
    "transformIgnorePatterns": [
    "setupFiles": [

Once you have all of your individual tests running and passing, you might see that Jest is still exiting with exit code 1. This occurred in React Native Gallery. After some searching, I discovered it was because some of the components our tests were rendering contained asynchronous calls and updates. This results in Jest throwing errors because it cannot ensure that all tasks such as renders, user events, or data fetching were completed and applied before the snapshot match assertion was made. To resolve this error, you’ll need to wrap the render of the component you wish to snapshot in React’s act() function. Then, following the act() call, make your snapshot assertion.

Here’s a code snippet of the test for React Native Gallery’s DateTimePicker page:

test('TimePicker Example Page', async () => {
  let tree;
  await act(() => {
    tree = create(<TimePickerExamplePage />).toJSON();

Concluding, in this article I’ve shared my process for adding Jest Snapshot testing to a React Native Windows application. Adding snapshot testing to React Native Gallery has helped us track visual changes to app components occur and validate that new releases of React Native Windows do not break community modules. Before leaving, I also wanted to share a couple of recommendations:

  1. The code for writing the tests is fairly straightforward; getting Jest configured correctly for your app can be where the challenge kicks in. If you’re working on an app that depends on community modules, consider adding tests for each module one at a time.
  2. Develop incrementally. Since Jest errors aren’t always clear, consider making granular changes between runs of the tests. That way it’ll be easier to diagnose what’s wrong. For example, start with a test case that simply renders a <View/> component. Make sure your base case runs successfully — then you can test as you go.

And here are a couple more pointers for some blockers I’ve hit while working on this:

  1. Attempting to run tests on the react-native-permissions module causes Jest to hang. As of now, we haven’t solved this issue, so the test is disabled.
  2. Some modules usage of the UIManager.getViewManagerConfig() API does not work well with Jest. UIManager returns undefined, even though the component is a part of core and thus should have already been transformed by Jest. As of now, we’ve had to disable tests for TrackPlayer and SketchCanvas.

You can also follow us on Twitter @ReactNativeMSFT to keep up to date on news, feature roadmaps, and more.


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