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Atlas Whoff
Atlas Whoff

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Playwright E2E Testing: Reliable Browser Tests for Next.js Apps

#typescript #testing #nextjs #webdev

Playwright E2E Testing: Reliable Browser Tests for Next.js Apps

Playwright is the most reliable browser automation tool for modern web apps.
Here's how to write tests that don't flake and actually catch real bugs.

Setup 

npm init playwright@latest
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This installs Playwright, creates playwright.config.ts, and adds test examples.

Configuration 

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

export default defineConfig({
  testDir: './tests/e2e',
  fullyParallel: true,
  forbidOnly: !!process.env.CI,
  retries: process.env.CI ? 2 : 0,
  workers: process.env.CI ? 1 : undefined,
  reporter: [['html'], ['list']],

  use: {
    baseURL: 'http://localhost:3000',
    trace: 'on-first-retry',
    screenshot: 'only-on-failure',
  },

  projects: [
    { name: 'chromium', use: { ...devices['Desktop Chrome'] } },
    { name: 'Mobile Safari', use: { ...devices['iPhone 13'] } },
  ],

  webServer: {
    command: 'npm run dev',
    url: 'http://localhost:3000',
    reuseExistingServer: !process.env.CI,
  },
})
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Writing Tests 

// tests/e2e/auth.spec.ts
import { test, expect } from '@playwright/test'

test.describe('Authentication', () => {
  test('user can sign up and log in', async ({ page }) => {
    await page.goto('/signup')

    await page.getByLabel('Email').fill('test@example.com')
    await page.getByLabel('Password').fill('password123')
    await page.getByRole('button', { name: 'Sign up' }).click()

    // Should redirect to dashboard
    await expect(page).toHaveURL('/dashboard')
    await expect(page.getByText('Welcome')).toBeVisible()
  })

  test('shows error for invalid credentials', async ({ page }) => {
    await page.goto('/login')
    await page.getByLabel('Email').fill('wrong@example.com')
    await page.getByLabel('Password').fill('wrongpassword')
    await page.getByRole('button', { name: 'Sign in' }).click()

    await expect(page.getByText('Invalid credentials')).toBeVisible()
  })
})
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Page Object Model 

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

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

  constructor(page: Page) {
    this.page = page
    this.emailInput = page.getByLabel('Email')
    this.passwordInput = page.getByLabel('Password')
    this.submitButton = page.getByRole('button', { name: 'Sign in' })
    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.submitButton.click()
  }
}

// In tests
const loginPage = new LoginPage(page)
await loginPage.goto()
await loginPage.login('test@example.com', 'password123')
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Authentication Fixtures 

// tests/fixtures.ts
import { test as base, Page } from '@playwright/test'

type Fixtures = {
  authenticatedPage: Page
}

export const test = base.extend<Fixtures>({
  authenticatedPage: async ({ page }, use) => {
    // Log in once, reuse session
    await page.goto('/login')
    await page.getByLabel('Email').fill(process.env.TEST_USER_EMAIL!)
    await page.getByLabel('Password').fill(process.env.TEST_USER_PASSWORD!)
    await page.getByRole('button', { name: 'Sign in' }).click()
    await page.waitForURL('/dashboard')

    await use(page)
  },
})

// All tests start already logged in
test('can create a project', async ({ authenticatedPage: page }) => {
  await page.goto('/projects/new')
  // ...
})
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API Mocking 

test('shows error when API fails', async ({ page }) => {
  // Intercept and mock the API
  await page.route('/api/projects', (route) => {
    route.fulfill({
      status: 500,
      body: JSON.stringify({ error: 'Server error' }),
    })
  })

  await page.goto('/projects')
  await expect(page.getByText('Something went wrong')).toBeVisible()
})
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Anti-Flakiness Rules 

// Use role-based locators (resilient to styling changes)
page.getByRole('button', { name: 'Submit' })  // GOOD
page.locator('.submit-btn')                    // BAD
page.locator('#submit')                        // BAD

// Wait for specific state, not arbitrary time
await expect(page.getByText('Saved')).toBeVisible()  // GOOD
await page.waitForTimeout(1000)                       // BAD
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