E2E Testing Patterns

# 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
```