Coverage

# V8 (built into Node — fast, no extra setup)
npm install -D @vitest/coverage-v8

# Istanbul (more accurate, slower)
npm install -D @vitest/coverage-istanbul

vitest run --coverage
vitest --coverage            # watch mode + coverage

// vitest.config.ts
import { defineConfig } from 'vitest/config'

export default defineConfig({
  test: {
    coverage: {
      provider: 'v8',           // 'istanbul' | 'v8'
      reporter: ['text', 'html', 'lcov', 'json'],
      reportsDirectory: './coverage',
      include: ['src/**/*.{ts,tsx}'],
      exclude: [
        'src/**/*.test.{ts,tsx}',
        'src/**/*.spec.{ts,tsx}',
        'src/test/**',
        'src/types/**',
      ],
      clean: true,              // clean before each run
    },
  },
})

coverage: {
  thresholds: {
    lines: 80,
    functions: 80,
    branches: 70,
    statements: 80,
    // Per-file thresholds:
    perFile: true,
    // Fail build if below thresholds:
    // (Vitest exits with code 1 if any threshold is unmet)
  },
}

reporter: ['text', 'html', 'lcov']

/* istanbul ignore next */
function debugOnly() {
  console.log('dev only')
}

/* istanbul ignore if */
if (process.env.DEBUG) {
  setup()
}

/* istanbul ignore else */
if (condition) {
  doThing()
} else {
  /* ignored branch */
}

/* v8 ignore next */
/* v8 ignore next 3 */       // ignore next N lines
/* v8 ignore start */
function notTested() { ... }
/* v8 ignore stop */

vitest run --coverage
open coverage/index.html

vitest --ui --coverage

# GitHub Actions example
- name: Run tests with coverage
  run: npx vitest run --coverage

- name: Upload to Codecov
  uses: codecov/codecov-action@v4
  with:
    files: ./coverage/lcov.info

coverage: {
  provider: 'istanbul',
  // Istanbul tracks logical branches including optional chaining
}

export default defineConfig({
  test: {
    coverage: { provider: 'v8' },
  },
  // Vite generates source maps automatically in test mode
})

// Replace .nycrc or c8 config with vitest.config.ts coverage block
coverage: {
  provider: 'istanbul', // matches nyc behavior most closely
  include: ['src/**'],
  exclude: ['**/*.test.ts'],
}