Coverage Threshold Requirements

This rule applies to files matching the patterns above.

# Coverage Threshold Requirements

## Rule
Projects MUST configure minimum coverage thresholds in jest.config.ts. Coverage gates must run in CI to prevent regressions.

## Configuration
```typescript
// jest.config.ts
export default {
  collectCoverageFrom: [
    'src/**/*.{ts,tsx}',
    '!src/**/*.d.ts',
    '!src/**/index.ts',
    '!src/**/__tests__/**',
    '!src/**/types/**',
    '!src/generated/**',
  ],

  coverageThreshold: {
    global: {
      statements: 80,
      branches: 75,
      functions: 80,
      lines: 80,
    },
    './src/services/': {
      statements: 90,
      branches: 85,
      functions: 90,
      lines: 90,
    },
    './src/utils/': {
      statements: 95,
      branches: 90,
      functions: 95,
      lines: 95,
    },
  },

  coverageReporters: ['text', 'text-summary', 'lcov', 'html'],
};
```

## Tier-Based Thresholds

| Code Category | Statement | Branch | Functions | Lines |
|---------------|-----------|--------|-----------|-------|
| Core business logic | 90% | 85% | 90% | 90% |
| Utilities / helpers | 95% | 90% | 95% | 95% |
| API routes / controllers | 80% | 75% | 80% | 80% |
| UI components | 70% | 65% | 70% | 70% |
| Configuration / setup | Excluded | Excluded | Excluded | Excluded |

## Good Coverage Exclusions
```typescript
// Exclude from coverage (legitimate reasons only)
collectCoverageFrom: [
  'src/**/*.ts',
  '!src/**/*.d.ts',        // Type definitions
  '!src/**/index.ts',      // Barrel exports
  '!src/generated/**',     // Auto-generated code
  '!src/**/__mocks__/**',  // Mock files
],
```

## Bad Coverage Exclusions
```typescript
// Never exclude these to inflate numbers
'!src/services/**',     // Core logic
'!src/utils/**',        // Utility functions
'!src/**/*.complex.ts', // "Complex" is not a valid reason
```

## CI Enforcement
```yaml
# In CI pipeline
- name: Run tests with coverage
  run: npx jest --coverage --ci
  # Jest will exit with code 1 if thresholds are not met
```

## Anti-Patterns
- Setting thresholds to 100% (impossible to maintain, leads to meaningless tests)
- Excluding files to meet thresholds instead of writing tests
- Tests that increase coverage but do not verify behavior
- Not tracking branch coverage (most important metric for logic correctness)