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E2e Testing Patterns

Master end-to-end testing with Playwright and Cypress to build reliable test suites that catch bugs, improve confidence, and enable fast deployment. Use when implementing E2E tests, debugging flaky tests, or establishing testing standards.

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Source: wshobson/agents Original Plugin: developer-essentials

E2E Testing Patterns

Build reliable, fast, and maintainable end-to-end test suites that provide confidence to ship code quickly and catch regressions before users do.

When to Use This Skill

  • Implementing end-to-end test automation
  • Debugging flaky or unreliable tests
  • Testing critical user workflows
  • Setting up CI/CD test pipelines
  • Testing across multiple browsers
  • Validating accessibility requirements
  • Testing responsive designs
  • Establishing E2E testing standards

Core Concepts

1. E2E Testing Fundamentals

What to Test with E2E:

  • Critical user journeys (login, checkout, signup)
  • Complex interactions (drag-and-drop, multi-step forms)
  • Cross-browser compatibility
  • Real API integration
  • Authentication flows

What NOT to Test with E2E:

  • Unit-level logic (use unit tests)
  • API contracts (use integration tests)
  • Edge cases (too slow)
  • Internal implementation details

2. Test Philosophy

The Testing Pyramid:

        /\
       /E2E\         ← Few, focused on critical paths
      /─────\
     /Integr\        ← More, test component interactions
    /────────\
   /Unit Tests\      ← Many, fast, isolated
  /────────────\

Best Practices:

  • Test user behavior, not implementation
  • Keep tests independent
  • Make tests deterministic
  • Optimize for speed
  • Use data-testid, not CSS selectors

Playwright Patterns

Setup and Configuration

TYPESCRIPT
// playwright.config.ts
import { defineConfig, devices } from '@playwright/test';

export default defineConfig({
    testDir: './e2e',
    timeout: 30000,
    expect: {
        timeout: 5000,
    },
    fullyParallel: true,
    forbidOnly: !!process.env.CI,
    retries: process.env.CI ? 2 : 0,
    workers: process.env.CI ? 1 : undefined,
    reporter: [
        ['html'],
        ['junit', { outputFile: 'results.xml' }],
    ],
    use: {
        baseURL: 'http://localhost:3000',
        trace: 'on-first-retry',
        screenshot: 'only-on-failure',
        video: 'retain-on-failure',
    },
    projects: [
        { name: 'chromium', use: { ...devices['Desktop Chrome'] } },
        { name: 'firefox', use: { ...devices['Desktop Firefox'] } },
        { name: 'webkit', use: { ...devices['Desktop Safari'] } },
        { name: 'mobile', use: { ...devices['iPhone 13'] } },
    ],
});

Pattern 1: Page Object Model

TYPESCRIPT
// pages/LoginPage.ts
import { Page, Locator } from '@playwright/test';

export class LoginPage {
    readonly page: Page;
    readonly emailInput: Locator;
    readonly passwordInput: Locator;
    readonly loginButton: Locator;
    readonly errorMessage: Locator;

    constructor(page: Page) {
        this.page = page;
        this.emailInput = page.getByLabel('Email');
        this.passwordInput = page.getByLabel('Password');
        this.loginButton = page.getByRole('button', { name: 'Login' });
        this.errorMessage = page.getByRole('alert');
    }

    async goto() {
        await this.page.goto('/login');
    }

    async login(email: string, password: string) {
        await this.emailInput.fill(email);
        await this.passwordInput.fill(password);
        await this.loginButton.click();
    }

    async getErrorMessage(): Promise<string> {
        return await this.errorMessage.textContent() ?? '';
    }
}

// Test using Page Object
import { test, expect } from '@playwright/test';
import { LoginPage } from './pages/LoginPage';

test('successful login', async ({ page }) => {
    const loginPage = new LoginPage(page);
    await loginPage.goto();
    await loginPage.login('user@example.com', 'password123');

    await expect(page).toHaveURL('/dashboard');
    await expect(page.getByRole('heading', { name: 'Dashboard' }))
        .toBeVisible();
});

test('failed login shows error', async ({ page }) => {
    const loginPage = new LoginPage(page);
    await loginPage.goto();
    await loginPage.login('invalid@example.com', 'wrong');

    const error = await loginPage.getErrorMessage();
    expect(error).toContain('Invalid credentials');
});

Pattern 2: Fixtures for Test Data

TYPESCRIPT
// fixtures/test-data.ts
import { test as base } from '@playwright/test';

type TestData = {
    testUser: {
        email: string;
        password: string;
        name: string;
    };
    adminUser: {
        email: string;
        password: string;
    };
};

export const test = base.extend<TestData>({
    testUser: async ({}, use) => {
        const user = {
            email: `test-${Date.now()}@example.com`,
            password: 'Test123!@#',
            name: 'Test User',
        };
        // Setup: Create user in database
        await createTestUser(user);
        await use(user);
        // Teardown: Clean up user
        await deleteTestUser(user.email);
    },

    adminUser: async ({}, use) => {
        await use({
            email: 'admin@example.com',
            password: process.env.ADMIN_PASSWORD!,
        });
    },
});

// Usage in tests
import { test } from './fixtures/test-data';

test('user can update profile', async ({ page, testUser }) => {
    await page.goto('/login');
    await page.getByLabel('Email').fill(testUser.email);
    await page.getByLabel('Password').fill(testUser.password);
    await page.getByRole('button', { name: 'Login' }).click();

    await page.goto('/profile');
    await page.getByLabel('Name').fill('Updated Name');
    await page.getByRole('button', { name: 'Save' }).click();

    await expect(page.getByText('Profile updated')).toBeVisible();
});

Pattern 3: Waiting Strategies

TYPESCRIPT
// ❌ Bad: Fixed timeouts
await page.waitForTimeout(3000);  // Flaky!

// ✅ Good: Wait for specific conditions
await page.waitForLoadState('networkidle');
await page.waitForURL('/dashboard');
await page.waitForSelector('[data-testid="user-profile"]');

// ✅ Better: Auto-waiting with assertions
await expect(page.getByText('Welcome')).toBeVisible();
await expect(page.getByRole('button', { name: 'Submit' }))
    .toBeEnabled();

// Wait for API response
const responsePromise = page.waitForResponse(
    response => response.url().includes('/api/users') && response.status() === 200
);
await page.getByRole('button', { name: 'Load Users' }).click();
const response = await responsePromise;
const data = await response.json();
expect(data.users).toHaveLength(10);

// Wait for multiple conditions
await Promise.all([
    page.waitForURL('/success'),
    page.waitForLoadState('networkidle'),
    expect(page.getByText('Payment successful')).toBeVisible(),
]);

Pattern 4: Network Mocking and Interception

TYPESCRIPT
// Mock API responses
test('displays error when API fails', async ({ page }) => {
    await page.route('**/api/users', route => {
        route.fulfill({
            status: 500,
            contentType: 'application/json',
            body: JSON.stringify({ error: 'Internal Server Error' }),
        });
    });

    await page.goto('/users');
    await expect(page.getByText('Failed to load users')).toBeVisible();
});

// Intercept and modify requests
test('can modify API request', async ({ page }) => {
    await page.route('**/api/users', async route => {
        const request = route.request();
        const postData = JSON.parse(request.postData() || '{}');

        // Modify request
        postData.role = 'admin';

        await route.continue({
            postData: JSON.stringify(postData),
        });
    });

    // Test continues...
});

// Mock third-party services
test('payment flow with mocked Stripe', async ({ page }) => {
    await page.route('**/api/stripe/**', route => {
        route.fulfill({
            status: 200,
            body: JSON.stringify({
                id: 'mock_payment_id',
                status: 'succeeded',
            }),
        });
    });

    // Test payment flow with mocked response
});

Cypress Patterns

Setup and Configuration

TYPESCRIPT
// cypress.config.ts
import { defineConfig } from 'cypress';

export default defineConfig({
    e2e: {
        baseUrl: 'http://localhost:3000',
        viewportWidth: 1280,
        viewportHeight: 720,
        video: false,
        screenshotOnRunFailure: true,
        defaultCommandTimeout: 10000,
        requestTimeout: 10000,
        setupNodeEvents(on, config) {
            // Implement node event listeners
        },
    },
});

Pattern 1: Custom Commands

TYPESCRIPT
// cypress/support/commands.ts
declare global {
    namespace Cypress {
        interface Chainable {
            login(email: string, password: string): Chainable<void>;
            createUser(userData: UserData): Chainable<User>;
            dataCy(value: string): Chainable<JQuery<HTMLElement>>;
        }
    }
}

Cypress.Commands.add('login', (email: string, password: string) => {
    cy.visit('/login');
    cy.get('[data-testid="email"]').type(email);
    cy.get('[data-testid="password"]').type(password);
    cy.get('[data-testid="login-button"]').click();
    cy.url().should('include', '/dashboard');
});

Cypress.Commands.add('createUser', (userData: UserData) => {
    return cy.request('POST', '/api/users', userData)
        .its('body');
});

Cypress.Commands.add('dataCy', (value: string) => {
    return cy.get(`[data-cy="${value}"]`);
});

// Usage
cy.login('user@example.com', 'password');
cy.dataCy('submit-button').click();

Pattern 2: Cypress Intercept

TYPESCRIPT
// Mock API calls
cy.intercept('GET', '/api/users', {
    statusCode: 200,
    body: [
        { id: 1, name: 'John' },
        { id: 2, name: 'Jane' },
    ],
}).as('getUsers');

cy.visit('/users');
cy.wait('@getUsers');
cy.get('[data-testid="user-list"]').children().should('have.length', 2);

// Modify responses
cy.intercept('GET', '/api/users', (req) => {
    req.reply((res) => {
        // Modify response
        res.body.users = res.body.users.slice(0, 5);
        res.send();
    });
});

// Simulate slow network
cy.intercept('GET', '/api/data', (req) => {
    req.reply((res) => {
        res.delay(3000);  // 3 second delay
        res.send();
    });
});

Advanced Patterns

Pattern 1: Visual Regression Testing

TYPESCRIPT
// With Playwright
import { test, expect } from '@playwright/test';

test('homepage looks correct', async ({ page }) => {
    await page.goto('/');
    await expect(page).toHaveScreenshot('homepage.png', {
        fullPage: true,
        maxDiffPixels: 100,
    });
});

test('button in all states', async ({ page }) => {
    await page.goto('/components');

    const button = page.getByRole('button', { name: 'Submit' });

    // Default state
    await expect(button).toHaveScreenshot('button-default.png');

    // Hover state
    await button.hover();
    await expect(button).toHaveScreenshot('button-hover.png');

    // Disabled state
    await button.evaluate(el => el.setAttribute('disabled', 'true'));
    await expect(button).toHaveScreenshot('button-disabled.png');
});

Pattern 2: Parallel Testing with Sharding

TYPESCRIPT
// playwright.config.ts
export default defineConfig({
    projects: [
        {
            name: 'shard-1',
            use: { ...devices['Desktop Chrome'] },
            grepInvert: /@slow/,
            shard: { current: 1, total: 4 },
        },
        {
            name: 'shard-2',
            use: { ...devices['Desktop Chrome'] },
            shard: { current: 2, total: 4 },
        },
        // ... more shards
    ],
});

// Run in CI
// npx playwright test --shard=1/4
// npx playwright test --shard=2/4

Pattern 3: Accessibility Testing

TYPESCRIPT
// Install: npm install @axe-core/playwright
import { test, expect } from '@playwright/test';
import AxeBuilder from '@axe-core/playwright';

test('page should not have accessibility violations', async ({ page }) => {
    await page.goto('/');

    const accessibilityScanResults = await new AxeBuilder({ page })
        .exclude('#third-party-widget')
        .analyze();

    expect(accessibilityScanResults.violations).toEqual([]);
});

test('form is accessible', async ({ page }) => {
    await page.goto('/signup');

    const results = await new AxeBuilder({ page })
        .include('form')
        .analyze();

    expect(results.violations).toEqual([]);
});

Best Practices

  1. Use Data Attributes: data-testid or data-cy for stable selectors
  2. Avoid Brittle Selectors: Don't rely on CSS classes or DOM structure
  3. Test User Behavior: Click, type, see - not implementation details
  4. Keep Tests Independent: Each test should run in isolation
  5. Clean Up Test Data: Create and destroy test data in each test
  6. Use Page Objects: Encapsulate page logic
  7. Meaningful Assertions: Check actual user-visible behavior
  8. Optimize for Speed: Mock when possible, parallel execution
TYPESCRIPT
// ❌ Bad selectors
cy.get('.btn.btn-primary.submit-button').click();
cy.get('div > form > div:nth-child(2) > input').type('text');

// ✅ Good selectors
cy.getByRole('button', { name: 'Submit' }).click();
cy.getByLabel('Email address').type('user@example.com');
cy.get('[data-testid="email-input"]').type('user@example.com');

Common Pitfalls

  • Flaky Tests: Use proper waits, not fixed timeouts
  • Slow Tests: Mock external APIs, use parallel execution
  • Over-Testing: Don't test every edge case with E2E
  • Coupled Tests: Tests should not depend on each other
  • Poor Selectors: Avoid CSS classes and nth-child
  • No Cleanup: Clean up test data after each test
  • Testing Implementation: Test user behavior, not internals

Debugging Failing Tests

TYPESCRIPT
// Playwright debugging
// 1. Run in headed mode
npx playwright test --headed

// 2. Run in debug mode
npx playwright test --debug

// 3. Use trace viewer
await page.screenshot({ path: 'screenshot.png' });
await page.video()?.saveAs('video.webm');

// 4. Add test.step for better reporting
test('checkout flow', async ({ page }) => {
    await test.step('Add item to cart', async () => {
        await page.goto('/products');
        await page.getByRole('button', { name: 'Add to Cart' }).click();
    });

    await test.step('Proceed to checkout', async () => {
        await page.goto('/cart');
        await page.getByRole('button', { name: 'Checkout' }).click();
    });
});

// 5. Inspect page state
await page.pause();  // Pauses execution, opens inspector

Resources

  • references/playwright-best-practices.md: Playwright-specific patterns
  • references/cypress-best-practices.md: Cypress-specific patterns
  • references/flaky-test-debugging.md: Debugging unreliable tests
  • assets/e2e-testing-checklist.md: What to test with E2E
  • assets/selector-strategies.md: Finding reliable selectors
  • scripts/test-analyzer.ts: Analyze test flakiness and duration