How do you test and ensure quality for web applications?

Comprehensive testing is built into our development process. Testing types include: unit testing (individual functions work correctly), integration testing (components work together), end-to-end testing (complete user journeys function), performance testing (load times, concurrent users), security testing (vulnerability scans, penetration testing), usability testing (real users provide feedback), cross-browser testing (works on Chrome, Safari, Firefox, Edge), mobile testing (responsive on all devices), and accessibility testing (works with screen readers). We use automated testing: Jest (JavaScript testing), Cypress (e-to-e testing), Lighthouse (performance audits), and OWASP ZAP (security scanning). Manual testing covers: edge cases, user flows, visual regression, and exploratory testing. Quality assurance process: development environment testing, staging environment review, user acceptance testing (UAT), then production deployment. We catch bugs before users encounter them. Test coverage aims for 80%+ of critical paths. Continuous integration runs tests automatically on every code change. Quality isn't accidental - it's systematically engineered.