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How to Run and Write Tests

Running Tests

This section is about testing your changes to React Native as a contributor. If you haven't yet, go through the steps to set up your development environment for building projects with native code.

JavaScript Tests

The simplest way to run the JavaScript test suite is by using the following command at the root of your React Native checkout:

yarn test

This will run tests using Jest.

You should also make sure your code passes Flow and lint tests:

yarn flow
yarn lint

iOS Tests

Follow the instructions in the packages/rn-tester directory.

Then, go back to the root of your React Native checkout and run yarn. This will set up your JavaScript dependencies.

At this point, you can run iOS tests by invoking the following script from the root of your React Native checkout:

./scripts/ test

You can also use Xcode to run iOS tests. Open RNTester/RNTesterPods.xcworkspace and run tests locally by pressing Command + U or selecting Product then Test from the menubar.

Xcode also allows running individual tests through its Test Navigator. You can also use Command + 6 shortcut.

note ensures your test environment is set up to run all tests. It also disables tests that are known to be flaky or broken. Keep this in mind when running tests using Xcode. If you see an unexpected failure, see if it's disabled in first.

iOS Podfile/Ruby tests

If you are making modifications to Podfile configurations then there are Ruby tests that can verify these.

To run the ruby tests:

cd scripts

Android Tests

The Android Unit tests do not run in an emulator, but they run on the JVM on your local machine.

To run the Android Unit tests, invoke the following script from the root of your React Native checkout:

./gradlew test

Writing Tests

Whenever you are fixing a bug or adding new functionality to React Native, it is a good idea to add a test that covers it. Depending on the change you're making, there are different types of tests that may be appropriate.

JavaScript Tests

The JavaScript tests can be found inside __test__ directories, colocated next to the files that are being tested. See TouchableHighlight-test.js for a basic example. You can also follow Jest's Testing React Native Apps tutorial to learn more.

iOS Integration Tests

React Native provides facilities to make it easier to test integrated components that require both native and JS components to communicate across the bridge.

The two main components are RCTTestRunner and RCTTestModule. RCTTestRunner sets up the React Native environment and provides facilities to run the tests as XCTestCases in Xcode (runTest:module is the simplest method). RCTTestModule is exported to JavaScript as NativeModules.TestModule.

The tests themselves are written in JS, and must call TestModule.markTestCompleted() when they are done, otherwise the test will timeout and fail.

Test failures are primarily indicated by throwing a JS exception. It is also possible to test error conditions with runTest:module:initialProps:expectErrorRegex: or runTest:module:initialProps:expectErrorBlock: which will expect an error to be thrown and verify the error matches the provided criteria.

See the following for example usage and integration points:

iOS Snapshot Tests

A common type of integration test is the snapshot test. These tests render a component, and verify snapshots of the screen against reference images using TestModule.verifySnapshot(), using the FBSnapshotTestCase library behind the scenes. Reference images are recorded by setting recordMode = YES on the RCTTestRunner, then running the tests.

Snapshots will differ slightly between 32 and 64 bit, and various OS versions, so it's recommended that you enforce tests are run with the correct configuration.

It's also highly recommended that all network data be mocked out, along with other potentially troublesome dependencies. See SimpleSnapshotTest for a basic example.

If you make a change that affects a snapshot test in a pull request, such as adding a new example case to one of the examples that is snapshotted, you'll need to re-record the snapshot reference image.

To do this, change recordMode flag to _runner.recordMode = YES; in RNTester/RNTesterSnapshotTests.m, re-run the failing tests, then flip record back to NO and submit/update your pull request and wait to see if the CircleCI build passes.

Android Unit Tests

It's a good idea to add an Android unit test whenever you are working on code that can be tested by Java/Kotlin code alone. The Android unit tests are located in packages/react-native/ReactAndroid/src/test/.

We recommend browsing through these to get an idea of what a good unit test might look like.

Continuous Testing

We use CircleCI to automatically run our open source tests. CircleCI will run these tests whenever a commit is added to a pull request, as a way to help maintainers understand whether a code change introduces a regression. The tests also run on commits to the main and *-stable branches in order to keep track of the health of these branches.

There's another set of tests that run within Meta's internal test infrastructure. Some of these tests are integration tests defined by internal consumers of React Native (e.g. unit tests for a React Native surface in the Facebook app).

These tests run on every commit to the copy of React Native hosted on Facebook's source control. They also run when a pull request is imported to Facebook's source control.

If one of these tests fail, you'll need someone at Meta to take a look. Since pull requests can only be imported by Meta employees, whoever imported the pull request should be able to facilitate any details.


Running CI tests locally: Most open source collaborators rely on CircleCI to see the results of these tests. If you'd rather verify your changes locally using the same configuration as CircleCI, CircleCI provides a command line interface with the ability to run jobs locally.


How do I upgrade the Xcode version used in CI tests?

When upgrading to a new version of Xcode, first make sure it is supported by CircleCI.

You will also need to update the test environment config to make sure tests run on an iOS Simulator that comes installed in the CircleCI machine.

This can also be found in CircleCI's Xcode version reference by clicking the desired version and looking under Runtimes.

You can then edit these two files:

  • .circleci/config.yml

    Edit the xcode: line under macos: (search for _XCODE_VERSION).

  • scripts/.tests.env

    Edit the IOS_TARGET_OS envvar to match the desired iOS Runtime.

If you intend to merge this change on GitHub, please make sure to notify a Meta employee as they'll need to update the value of _XCODE_VERSION used in the internal Sandcastle RN OSS iOS test in when they import your pull request.