Frontend Testing

Frontend Testing

CRITICAL: The description field above controls when Claude auto-loads this skill.

Overview

Provides comprehensive frontend testing patterns for React/Next.js applications including:

• Component testing with Jest/Vitest + React Testing Library
• Visual regression testing with Playwright
• Accessibility testing with axe-core
• Performance testing with Lighthouse
• Test infrastructure setup and configuration
• Coverage analysis and reporting

Security Requirements

All code examples and templates in this skill follow strict security rules:

CRITICAL: Reference @docs/security/SECURITY-RULES.md

• Placeholders only, never real credentials
• Environment variable references in code
• .gitignore protection for secrets
• Setup documentation for key acquisition

Instructions

1. Initialize Frontend Test Infrastructure

Use scripts/init-frontend-tests.sh to set up comprehensive frontend testing:

bash scripts/init-frontend-tests.sh [project-path]

This will:

• Detect testing framework (Jest or Vitest)
• Install React Testing Library dependencies
• Install Playwright for visual regression
• Install axe-core for accessibility testing
• Install Lighthouse for performance testing
• Create test configuration files
• Set up test utilities and helpers
• Create initial test examples

2. Run Component Tests

Use scripts/run-component-tests.sh to execute component tests:

bash scripts/run-component-tests.sh [test-pattern] [options]

Options:

• Test pattern: Specific test file or glob pattern
• Options: --coverage, --watch, --updateSnapshot

This will:

• Run Jest or Vitest tests
• Execute React Testing Library tests
• Generate coverage reports
• Report pass/fail status

3. Run Visual Regression Tests

Use scripts/run-visual-regression.sh for visual testing:

bash scripts/run-visual-regression.sh [test-pattern] [update-snapshots]

This will:

• Run Playwright visual regression tests
• Compare against baseline snapshots
• Generate diff images for failures
• Update snapshots if requested
• Mask dynamic content automatically

4. Run Accessibility Tests

Use scripts/run-accessibility-tests.sh for a11y testing:

bash scripts/run-accessibility-tests.sh [test-pattern]

This will:

• Run axe-core accessibility tests via Playwright
• Check ARIA attributes
• Validate keyboard navigation
• Test color contrast
• Report WCAG violations

5. Run Performance Tests

Use scripts/run-performance-tests.sh for performance testing:

bash scripts/run-performance-tests.sh [url] [options]

This will:

• Run Lighthouse audits
• Check Core Web Vitals (LCP, FID, CLS)
• Analyze bundle size
• Monitor render performance
• Generate performance reports

6. Generate Coverage Report

Use scripts/generate-coverage-report.sh to aggregate coverage:

bash scripts/generate-coverage-report.sh [output-dir]

This will:

• Aggregate coverage from all test types
• Generate HTML coverage report
• Calculate coverage percentages
• Identify untested files
• Report coverage by test type

Available Scripts

• init-frontend-tests.sh: Initialize complete frontend testing infrastructure
• run-component-tests.sh: Execute Jest/Vitest component tests
• run-visual-regression.sh: Execute Playwright visual regression tests
• run-accessibility-tests.sh: Execute axe-core accessibility tests
• run-performance-tests.sh: Execute Lighthouse performance tests
• generate-coverage-report.sh: Aggregate coverage data and generate reports

Templates

Configuration Files

• jest.config.js: Jest configuration for React/Next.js
• vitest.config.ts: Vitest configuration for React/Next.js
• test-utils.ts: Shared test utilities and custom renderers
• playwright.visual.config.ts: Playwright config for visual regression
• playwright.a11y.config.ts: Playwright config for accessibility testing

Test Templates

• component-test.spec.tsx: Component test template with RTL
• visual-regression.spec.ts: Visual regression test template
• accessibility.spec.ts: Accessibility test template
• performance.spec.ts: Performance test template with Lighthouse

Utility Templates

• setup-tests.ts: Test setup and global mocks
• test-helpers.ts: Custom test helper functions
• mock-factories.ts: Mock data factories

Examples

• button-component-test.tsx: Real component test for Button component
• login-page-visual.spec.ts: Real visual regression test for login page
• form-accessibility.spec.ts: Real accessibility test for form components
• homepage-performance.spec.ts: Real performance test for homepage

Component Testing Patterns

React Testing Library Pattern

import { render, screen, userEvent } from '@testing-library/react';
import { Button } from './Button';

test('renders and handles click', async () => {
  const handleClick = jest.fn();
  render(<Button me</Button>);

  const button = screen.getByRole('button', { name: /click me/i });
  await userEvent.click(button);

  expect(handleClick).toHaveBeenCalledTimes(1);
});

Testing Patterns

• Rendering: Use render() from RTL
• Querying: Prefer getByRole, getByText, getByLabelText
• User Interaction: Use userEvent for realistic interactions
• Assertions: Assert on visible behavior, not implementation
• Async: Use waitFor for async operations

Visual Regression Testing Patterns

Baseline Creation

import { test, expect } from '@playwright/test';

test('homepage visual regression', async ({ page }) => {
  await page.goto('http://localhost:3000');
  await page.waitForLoadState('networkidle');

  // Mask dynamic content
  await page.locator('.timestamp').evaluate(el => el.style.visibility = 'hidden');

  await expect(page).toHaveScreenshot('homepage.png');
});

Best Practices

• Create stable baselines
• Use consistent viewport sizes
• Mask dynamic content (dates, random IDs, animations)
• Wait for network idle
• Test critical user paths

Accessibility Testing Patterns

axe-core Integration

import { test, expect } from '@playwright/test';
import { injectAxe, checkA11y } from 'axe-playwright';

test('form accessibility', async ({ page }) => {
  await page.goto('http://localhost:3000/form');
  await injectAxe(page);

  await checkA11y(page, null, {
    detailedReport: true,
    detailedReportOptions: {
      html: true
    }
  });
});

WCAG Compliance

• Check ARIA attributes
• Validate keyboard navigation (Tab, Enter, Escape)
• Test screen reader compatibility
• Verify color contrast
• Ensure focus management

Performance Testing Patterns

Lighthouse Integration

import { test } from '@playwright/test';
import { playAudit } from 'playwright-lighthouse';

test('homepage performance', async ({ page }) => {
  await page.goto('http://localhost:3000');

  await playAudit({
    page,
    thresholds: {
      performance: 90,
      accessibility: 90,
      'best-practices': 90,
      seo: 90,
      'first-contentful-paint': 2000,
      'largest-contentful-paint': 3000,
      'cumulative-layout-shift': 0.1,
    },
    port: 9222,
  });
});

Core Web Vitals

• LCP: Largest Contentful Paint < 2.5s
• FID: First Input Delay < 100ms
• CLS: Cumulative Layout Shift < 0.1
• TTFB: Time to First Byte < 800ms

Test Organization

Directory Structure

tests/
├── unit/                    # Component unit tests
│   ├── Button.spec.tsx
│   └── Form.spec.tsx
├── integration/             # Component integration tests
│   └── AuthFlow.spec.tsx
├── visual/                  # Visual regression tests
│   ├── homepage.spec.ts
│   └── dashboard.spec.ts
├── a11y/                   # Accessibility tests
│   ├── navigation.spec.ts
│   └── forms.spec.ts
├── performance/            # Performance tests
│   └── critical-pages.spec.ts
└── utils/                  # Test utilities
    ├── test-utils.ts
    ├── mock-factories.ts
    └── test-helpers.ts

Coverage Targets

Component Tests (Jest/Vitest + RTL)

• Target: 80%+ code coverage
• Focus: Business logic, user interactions, edge cases
• Metrics: Lines, branches, functions, statements

Visual Regression Tests (Playwright)

• Target: All pages, critical components
• Focus: UI stability, design system compliance
• Metrics: Pages tested, components tested

Accessibility Tests (axe-core)

• Target: All pages, interactive components
• Focus: WCAG 2.1 AA compliance
• Metrics: Violations found, compliance score

Performance Tests (Lighthouse)

• Target: All pages, critical user flows
• Focus: Core Web Vitals, loading performance
• Metrics: Performance score, web vitals

CI/CD Integration

GitHub Actions Example

- name: Run Frontend Tests
  run: |
    npm run test:component -- --coverage
    npm run test:visual
    npm run test:a11y
    npm run test:performance

- name: Upload Coverage
  uses: codecov/codecov-action@v3
  with:
    files: ./coverage/coverage-final.json

Test Execution Order

1. Component tests (fastest, run first)
2. Visual regression tests
3. Accessibility tests
4. Performance tests (slowest, run last)

Debugging Tips

Component Test Debugging

• Use screen.debug() to print DOM
• Use screen.logTestingPlaygroundURL() for selector help
• Run with --watch for iterative debugging
• Use test.only to focus on single test

Visual Test Debugging

• Compare diff images in test-results/
• Update snapshots carefully
• Check for timing issues (waitForLoadState)
• Verify dynamic content is masked

Accessibility Test Debugging

• Review detailed axe reports
• Use browser DevTools accessibility panel
• Test with actual screen readers
• Check keyboard navigation manually

Performance Test Debugging

• Review Lighthouse HTML reports
• Check network waterfall
• Analyze bundle size
• Profile render performance

Requirements

• Node.js 18+ installed
• React 18+ or Next.js 13+
• Jest 29+ or Vitest 1.0+ for component tests
• Playwright 1.40+ for visual/a11y tests
• @axe-core/playwright for accessibility
• playwright-lighthouse for performance
• @testing-library/react for component testing
• @testing-library/user-event for interactions

Best Practices

Component Testing

• Test user behavior, not implementation
• Use accessible queries (getByRole, getByLabelText)
• Avoid testing implementation details
• Keep tests independent and isolated
• Mock external dependencies

Visual Regression

• Create stable baselines
• Mask dynamic content
• Use consistent viewport sizes
• Test critical UI states
• Update snapshots intentionally

Accessibility

• Test with keyboard only
• Check ARIA attributes
• Validate focus management
• Test with screen readers
• Follow WCAG 2.1 AA guidelines

Performance

• Test on realistic devices/connections
• Monitor Core Web Vitals
• Check bundle sizes
• Profile render performance
• Set realistic thresholds

Common Patterns

Testing Forms

test('form submission', async () => {
  render(<ContactForm />);

  await userEvent.type(screen.getByLabelText(/name/i), 'John Doe');
  await userEvent.type(screen.getByLabelText(/email/i), 'john@example.com');
  await userEvent.click(screen.getByRole('button', { name: /submit/i }));

  expect(await screen.findByText(/success/i)).toBeInTheDocument();
});

Testing Async Data

test('loads and displays data', async () => {
  render(<UserProfile userId="123" />);

  expect(screen.getByText(/loading/i)).toBeInTheDocument();

  const userName = await screen.findByText(/john doe/i);
  expect(userName).toBeInTheDocument();
});

Testing Error States

test('displays error message', async () => {
  server.use(
    http.get('/api/user', () => {
      return HttpResponse.json({ error: 'Not found' }, { status: 404 });
    })
  );

  render(<UserProfile userId="999" />);

  expect(await screen.findByText(/error/i)).toBeInTheDocument();
});

Next Steps

1. Run init-frontend-tests.sh to set up infrastructure
2. Generate component tests for existing components
3. Create visual regression baselines
4. Run accessibility audits
5. Set up performance monitoring
6. Integrate with CI/CD pipeline
7. Monitor coverage and quality metrics