@wshobson/e2e-testing-patterns

---
name: e2e-testing-patterns
description: 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.
---

# 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
```