Integration tests cover critical workflows

Unit tests verify individual functions in isolation; they cannot catch bugs that only appear when multiple units interact — a misconfigured middleware, a mismatched API contract between frontend and backend, or an authentication check that passes locally but fails in a real browser session. ISO 25010:2011 §6.5.5 requires verification at the workflow level. For user-facing applications, the most costly bugs are the ones that break the critical path — signup, login, checkout — while all the unit tests are green.

Start with the most important user journey. For a web app with authentication, a Playwright test covers the full stack in one shot:

// tests/auth-flow.test.ts
import { test, expect } from '@playwright/test'

test('user can sign in and access dashboard', async ({ page }) => {
  await page.goto('/login')
  await page.fill('[name=email]', 'test@example.com')
  await page.fill('[name=password]', 'testpassword')
  await page.click('[type=submit]')
  await expect(page).toHaveURL('/dashboard')
  await expect(page.locator('h1')).toContainText('Welcome')
})

For API routes, use supertest or route-handler integration tests. Aim for at least three workflow-level tests covering: authentication, primary data submission, and the application's core action.