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The React Native repo has several tests you can run to verify you haven't caused a regression with your PR. These tests are run with the Travis continuous integration system, and will automatically post the results to your PR. You can also run them locally with cmd+U in the IntegrationTest and UIExplorer apps in Xcode. You can run the jest tests via npm test on the command line. We don't have great test coverage yet, however, so most changes will still require significant manual verification, but we would love it if you want to help us increase our test coverage!
Jest tests are JS-only tests run on the command line with node. The tests themselves live in the __tests__ directories of the files they test, and there is a large emphasis on aggressively mocking out functionality that is not under test for failure isolation and maximum speed. You can run the existing React Native jest tests with npm test from the react-native root, and we encourage you to add your own tests for any components you want to contribute to. See getImageSource-test.js for a basic example.
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 ReactNative environment and provides facilities to run the tests as XCTestCases in Xcode (runTest:module is the simplest method). RCTTestModule is exported to JS via NativeModules as 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 an 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 IntegrationTestHarnessTest.js and IntegrationTestsTests.m for example usage.
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.