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e2e-testing-patterns
Master end-to-end testing with Playwright and Cypress to build
# 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\ β Few, focused on critical paths
/βββββ\
/Integr\ β More, test component interactions
/ββββββββ\
/Unit Tests\ β Many, fast, isolated
/ββββββββββββ\
```
**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
// β Bad: Fixed timeouts
await page.waitForTimeout(3000); // Flaky!
// β
Good: Wait for specific conditions
await page.waitForLoadState("networkidle");
await page.waitForURL("/dashboard");
await page.waitForSelector('[data-testid="user-profile"]');
// β
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
// β Bad selectors
cy.get(".btn.btn-primary.submit-button").click();
cy.get("div > form > div:nth-child(2) > input").type("text");
// β
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
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