// Unit test
describe('CartService', () => {
  it('calculates total correctly', () => {
    const cart = new CartService();
    cart.addItem({ price: 10, quantity: 2 });
    expect(cart.getTotal()).toBe(20);
  });
});

// E2E test (Detox)
describe('Login flow', () => {
  beforeEach(async () => {
    await device.reloadReactNative();
  });

  it('should login successfully', async () => {
    await element(by.id('email-input')).typeText('user@example.com');
    await element(by.id('password-input')).typeText('password123');
    await element(by.id('login-button')).tap();
    await expect(element(by.id('dashboard'))).toBeVisible();
  });
});

iOS (XCTest)

func testLoginSuccess() {
    let app = XCUIApplication()
    app.launch()

    app.textFields["email"].tap()
    app.textFields["email"].typeText("user@example.com")
    app.secureTextFields["password"].typeText("password123")
    app.buttons["Login"].tap()

    XCTAssertTrue(app.staticTexts["Welcome"].exists)
}

Android (Espresso)

@Test
fun loginSuccess() {
    onView(withId(R.id.email)).perform(typeText("user@example.com"))
    onView(withId(R.id.password)).perform(typeText("password123"))
    onView(withId(R.id.loginButton)).perform(click())
    onView(withId(R.id.dashboard)).check(matches(isDisplayed()))
}

Best Practices

Test business logic first (unit tests)
Mock external dependencies
Test both success and failure paths
Automate critical user flows
Maintain >80% code coverage
Test on real devices periodically

Avoid

Testing implementation details
Hardcoded test data
Interdependent tests
Skipping error case testing

Level	Tools	Coverage
Unit	Jest, XCTest, JUnit	70%
Integration	Detox, Espresso	20%
E2E	Appium, Detox	10%

mobile-app-testing

Mobile App Testing

Testing Pyramid

React Native (Jest + Detox)