Code Coverage Analysis

Code Coverage Analysis

This skill makes an AI agent configure coverage collection correctly (Istanbul instrumentation or V8 native coverage), set thresholds that fail builds, read coverage reports to find genuinely untested branches, and exclude generated or config code so the numbers mean something. Trigger it when a user asks "what is our coverage", wants a coverage gate in CI, or when a coverage/ directory, --coverage flag, .nycrc, or coverageThreshold appears in the project.

Core Principles

1. Branch coverage is the number that matters. Line coverage marks a line as hit even if only one of its outcomes ran. A guard clause if (a && b) return x; can be 100 percent line-covered with three of its four branch outcomes untested.
2. Coverage is a gap detector, not a quality score. 95 percent coverage with assertion-free tests proves nothing. Use coverage to find untested code paths; use mutation testing to check assertion strength.
3. Thresholds must fail the build. A coverage report nobody reads is decoration. Wire coverageThreshold (Jest), thresholds (Vitest), or --check-coverage (nyc/c8) so CI goes red on regression.
4. Set thresholds at current reality, then ratchet. Dropping a 90 percent global gate onto a 60 percent codebase makes the team delete the gate. Start at today's number minus 1, raise it as coverage improves.
5. Measure all files, not just imported ones. By default some tools only report files touched by tests, so a completely untested module is invisible. Enable all: true (nyc), coverage.all (Vitest), or a broad collectCoverageFrom (Jest).
6. Exclude generated code explicitly. Protobuf stubs, GraphQL codegen, migrations, and *.d.ts inflate or deflate numbers randomly. Exclude them in config, never by sprinkling ignore comments through generated files.

Setup and Patterns

1. Jest: collection, thresholds, and per-directory gates

// jest.config.js
module.exports = {
  preset: 'ts-jest',
  collectCoverage: true,
  coverageProvider: 'v8',
  collectCoverageFrom: [
    'src/**/*.{ts,tsx}',
    '!src/**/*.d.ts',
    '!src/**/__generated__/**',
    '!src/**/*.stories.tsx',
    '!src/test-utils/**',
  ],
  coverageReporters: ['text', 'lcov', 'json-summary'],
  coverageThreshold: {
    global: {
      branches: 80,
      functions: 85,
      lines: 90,
      statements: 90,
    },
    // Money-handling code gets a stricter gate
    './src/lib/payments/': {
      branches: 95,
      lines: 98,
    },
  },
};