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Testing Interview Guide

Common Interview Questions

Unit Testing

Q: How do you test React components?

  • Use React Testing Library
  • Test user behavior, not implementation
  • Query by role, text, or test-id
  • Avoid testing internal state

Q: What's the difference between unit and integration tests?

  • Unit: Test individual functions/components in isolation
  • Integration: Test multiple components working together
  • Unit tests are faster, integration tests catch more bugs

Q: How do you mock API calls in tests?

  • Use MSW (Mock Service Worker)
  • Use Jest mocks
  • Mock fetch/axios
  • Return predictable test data

E2E Testing

Q: What tools do you use for E2E testing?

  • Playwright (modern, fast, reliable)
  • Cypress (great DX, developer-friendly)
  • Selenium (industry standard)

Q: How do you handle flaky tests?

  • Use proper wait strategies
  • Avoid brittle selectors
  • Use data-testid attributes
  • Retry logic for network issues
  • Isolate tests properly

Q: What's your strategy for E2E test maintenance?

  • Page Object Model
  • Reusable test utilities
  • Regular test reviews
  • Remove obsolete tests
  • Keep tests focused

Performance Testing

Q: How do you measure frontend performance?

  • Core Web Vitals (LCP, FID, CLS)
  • Lighthouse scores
  • Real User Monitoring (RUM)
  • Performance budgets

Q: What metrics do you track?

  • LCP < 2.5s
  • FID < 100ms
  • CLS < 0.1
  • TTFB < 600ms
  • Bundle size

Q: How do you test under load?

  • Load testing tools (k6, Artillery)
  • Stress testing
  • Spike testing
  • Monitor metrics under load

A/B Testing

Q: How do you implement A/B tests?

  • Variant assignment (consistent)
  • Traffic splitting
  • Analytics tracking
  • Statistical analysis
  • Rollout strategy

Q: How do you ensure statistical significance?

  • Sufficient sample size
  • Run for adequate duration
  • 95% confidence level
  • Account for multiple testing

TDD

Q: What is TDD?

  • Write tests before code
  • Red-Green-Refactor cycle
  • Tests drive design
  • Confidence in code

Q: When do you use TDD?

  • Complex logic
  • Critical features
  • APIs
  • Bug fixes
  • Not always for simple code

Security Testing

Q: How do you test for security vulnerabilities?

  • Automated scanning (Snyk, OWASP ZAP)
  • Dependency scanning
  • Penetration testing
  • Security code reviews
  • Regular audits

Q: What vulnerabilities do you test for?

  • XSS
  • CSRF
  • SQL Injection
  • Authentication bypass
  • Sensitive data exposure
  • Insecure dependencies

Testing Best Practices Summary

DO's

  • ✅ Test behavior, not implementation
  • ✅ Keep tests fast and isolated
  • ✅ Use descriptive test names
  • ✅ Test edge cases and errors
  • ✅ Maintain good coverage
  • ✅ Mock external dependencies
  • ✅ Keep tests maintainable

DON'Ts

  • ❌ Don't test implementation details
  • ❌ Don't over-mock
  • ❌ Don't write slow tests
  • ❌ Don't ignore flaky tests
  • ❌ Don't test third-party code
  • ❌ Don't write dependent tests

Testing Pyramid

        /\
       /  \      E2E Tests (Few, Critical Paths)
      /____\
     /      \    Integration Tests (Some, Key Flows)
    /________\
   /          \  Unit Tests (Many, Fast, Isolated)
  /____________\

Key Metrics

  • Code Coverage - Aim for >80%
  • Test Execution Time - Keep fast
  • Flaky Test Rate - Minimize
  • Test Maintenance - Regular reviews

Testing Checklist

Before Deployment

  • All tests passing
  • Code coverage >80%
  • E2E tests for critical paths
  • Performance benchmarks met
  • Security tests passing
  • No flaky tests
  • Test documentation updated