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) => {