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name mobile-testing
description Set up unit and integration testing for React Native/Expo or Flutter apps. Covers Jest, React Native Testing Library, flutter_test, snapshot testing, mocking native modules, and test organization. Use when the user wants to add or improve tests for components, hooks, or business logic.
standards-version 1.9.0

Mobile Testing

Trigger

Use this skill when the user:

  • Wants to add unit or integration tests to their app
  • Asks about Jest, React Native Testing Library, or flutter_test
  • Needs to test components, hooks, utilities, or state management
  • Mentions "testing", "unit test", "snapshot test", "test coverage", or "mock"
  • Wants help structuring test files or writing assertions

Required Inputs

  • Framework: Expo (React Native) or Flutter
  • Test scope: components, hooks, utilities, or all
  • Existing test setup: none, partial, or already configured

Workflow

  1. Install test dependencies (Expo). Expo projects need Jest and React Native Testing Library:

    npx expo install jest jest-expo @testing-library/react-native @testing-library/jest-native

    Add the Jest config to package.json:

    {
  "jest": {
    "preset": "jest-expo",
    "setupFilesAfterSetup": ["@testing-library/jest-native/extend-expect"],
    "transformIgnorePatterns": [
      "node_modules/(?!((jest-)?react-native|@react-native(-community)?)|expo(nent)?|@expo(nent)?/.*|@expo-google-fonts/.*|react-navigation|@react-navigation/.*|@sentry/react-native|native-base|react-native-svg)"
    ]
  }
}

  2. Install test dependencies (Flutter). Flutter ships with flutter_test. For additional matchers:

    dev_dependencies:
  flutter_test:
    sdk: flutter
  mocktail: ^1.0.4

  3. Organize test files. Follow the convention for each framework:

    Framework Convention Example
    Expo __tests__/ComponentName.test.tsx next to source components/__tests__/Avatar.test.tsx
    Expo (colocated) ComponentName.test.tsx in the same directory components/Avatar.test.tsx
    Flutter Mirror lib/ structure under test/ test/widgets/avatar_test.dart

  4. Write a component test (Expo). Test rendering and interaction:

    import { render, screen, fireEvent } from "@testing-library/react-native";
import { Counter } from "../Counter";

describe("Counter", () => {
  it("renders the initial count", () => {
    render(<Counter initialCount={5} />);
    expect(screen.getByText("5")).toBeTruthy();
  });

  it("increments on press", () => {
    render(<Counter initialCount={0} />);
    fireEvent.press(screen.getByRole("button", { name: "Increment" }));
    expect(screen.getByText("1")).toBeTruthy();
  });
});

  5. Write a widget test (Flutter).

    import 'package:flutter_test/flutter_test.dart';
import 'package:my_app/widgets/counter.dart';

void main() {
  testWidgets('Counter increments on tap', (tester) async {
    await tester.pumpWidget(const MaterialApp(home: Counter(initialCount: 0)));
    expect(find.text('0'), findsOneWidget);

    await tester.tap(find.byIcon(Icons.add));
    await tester.pump();
    expect(find.text('1'), findsOneWidget);
  });
}

  6. Mock native modules. Many Expo modules need mocks in jest.setup.js:

    jest.mock("expo-camera", () => ({
  Camera: "Camera",
  useCameraPermissions: () => [{ granted: true }, jest.fn()],
}));

jest.mock("@react-native-async-storage/async-storage", () =>
  require("@react-native-async-storage/async-storage/jest/async-storage-mock")
);

    Reference the setup file in your Jest config:

    {
  "jest": {
    "setupFiles": ["./jest.setup.js"]
  }
}

  7. Test custom hooks. Use renderHook from React Native Testing Library:

    import { renderHook, act } from "@testing-library/react-native";
import { useCounter } from "../useCounter";

it("increments the counter", () => {
  const { result } = renderHook(() => useCounter(0));
  act(() => result.current.increment());
  expect(result.current.count).toBe(1);
});

  8. Snapshot testing. Capture component output for regression detection:

    import { render } from "@testing-library/react-native";
import { ProfileCard } from "../ProfileCard";

it("matches snapshot", () => {
  const tree = render(<ProfileCard name="Alice" />);
  expect(tree.toJSON()).toMatchSnapshot();
});

    Update snapshots when intentional changes are made: npx jest --updateSnapshot.

  9. Run tests. Execute the suite and check results:

    # Expo
npx jest
npx jest --coverage
npx jest --watch

# Flutter
flutter test
flutter test --coverage

Key References

Example Interaction

User: "I want to add tests to my Expo app. I have components and custom hooks but no tests yet."

Agent:

  1. Installs jest-expo, @testing-library/react-native, and @testing-library/jest-native with mobile_installDependency
  2. Adds Jest config to package.json with the jest-expo preset and transform ignore patterns
  3. Creates jest.setup.js with mocks for native modules used in the project
  4. Generates a test file for a key component using mobile_generateTestFile
  5. Writes a renderHook test for the most-used custom hook
  6. Runs mobile_runTests to verify everything passes
  7. Suggests adding --coverage to track test coverage over time

MCP Usage

Step MCP Tool Description
Install test libs mobile_installDependency Install jest-expo, @testing-library/react-native
Generate test file mobile_generateTestFile Scaffold test boilerplate for an existing component
Run test suite mobile_runTests Execute tests and report pass/fail summary
Verify build mobile_checkBuildHealth Ensure test setup does not break the build

Common Pitfalls

  1. Missing transform ignore patterns - Jest fails on untranspiled node_modules. The transformIgnorePatterns array must include every RN-related package that ships untranspiled ESM or JSX.
  2. Testing implementation details - Test behavior (what the user sees), not internal state. Query by text, role, or testID, not by component internals.
  3. Stale snapshots - Snapshots become noise if updated blindly. Review every snapshot diff before committing. Delete snapshots that test layout rather than logic.
  4. Not mocking native modules - Tests crash if native modules like expo-camera or expo-notifications are not mocked. Add mocks in jest.setup.js.
  5. Async state not awaited - Use waitFor or findBy* queries for components that update state asynchronously. Bare getBy* queries miss async renders.
  6. Flutter test imports - Always import from package:your_app/... in tests, not relative paths. The test runner resolves packages differently.

See Also