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Testing Best Practices

Testing Best Practices

Introduction

This topic summarizes the key practices for testing Next.js applications effectively.

Testing Strategy

flowchart TD
    A[Test Strategy] --> B{Fast test?}
    B -->|Yes| C[Unit test]
    B -->|No| D{Feature test?}
    D -->|Yes| E[Integration test]
    D -->|No| F{Critical flow?}
    F -->|Yes| G[E2E test]
    F -->|No| H[Manual QA]

What to Test

Priority	What	Test Type
High	Utility functions, helpers	Unit
High	API routes, Server Actions	Integration
High	Authentication flow	E2E
Medium	Form submissions	Integration
Medium	Rendering with various states	Unit
Low	Styling, layout	Visual

Writing Good Tests

// ✅ Good test — describes behavior
it('redirects unauthenticated users to login', async () => {
  const response = await fetch('http://localhost:3000/dashboard')
  expect(response.redirected).toBe(true)
  expect(response.url).toContain('/login')
})

// ❌ Not ideal — tests implementation
it('calls getServerSession', async () => {
  const spy = vi.spyOn(auth, 'getServerSession')
  await DashboardPage()
  expect(spy).toHaveBeenCalled()
})

CI Integration

name: Tests
on: [push, pull_request]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - uses: actions/setup-node@v4
      - run: npm ci
      - run: npx vitest run         # Unit + Integration
      - run: npx playwright test    # E2E

Common Mistakes

Not running tests in CI — Tests only catch issues if they’re run. Integrate them into your CI pipeline.
Ignoring flaky tests — A flaky test loses its value. Fix or remove it.
Testing only happy paths — Test error states, empty states, and edge cases.

Best Practices

Run unit and integration tests on every commit
Run E2E tests before deployment
Keep tests fast — slow tests don’t get run as often
Test behavior, not implementation
Use descriptive test names
Clean up test data between runs

Summary

A good testing strategy balances speed and confidence. Use unit tests for fast feedback, integration tests for feature verification, and a few E2E tests for critical flows. Run them in CI and keep them fast.