feat: Add Node.js built-in test coverage per package

[adrianbrowning]

Summary

Add code coverage reporting to each package in the monorepo using Node.js's built-in test runner (node:test) and the --experimental-test-coverage flag — no extra dependencies required.

Background

Node.js 20+ includes a built-in coverage tool via --experimental-test-coverage. Since the repo already uses node:test, this is a zero-dependency way to get line, branch, and function coverage per package.

Reference: https://nodejs.org/api/test.html#collecting-code-coverage

---

Tasks

Per-package (packages/*/package.json)

Add the following scripts to each package:

"scripts": {
  "test": "node --experimental-strip-types --test",
  "test:coverage": "node --experimental-strip-types --test --experimental-test-coverage --test-coverage-include='src/**/*.ts' --test-coverage-exclude='src/**/*.test.ts'",
  "test:coverage:ci": "node --experimental-strip-types --test --experimental-test-coverage --test-reporter=spec --test-reporter-destination=stdout --test-reporter=lcov --test-reporter-destination=lcov.info --test-coverage-include='src/**/*.ts'"
}

Use --experimental-strip-types for TypeScript support (Node 22.6+), or --import tsx/esm for older Node versions.

Root package.json

"scripts": {
  "test": "pnpm -r test",
  "test:coverage": "pnpm -r test:coverage"
}

Optional: Coverage thresholds

Enforce minimum coverage levels per package to prevent regressions:

--test-coverage-lines=80 --test-coverage-branches=70 --test-coverage-functions=80

Optional: node.test.config.mjs per package

For cleaner config (requires Node 22.8+):

// packages/*/node.test.config.mjs
export default {
  coverage: true,
  coverageIncludeGlobs: ['src/**/*.ts'],
  coverageExcludeGlobs: ['src/**/*.test.ts', '**/*.d.ts'],
  lineCoverage: 80,
  branchCoverage: 70,
  functionCoverage: 80,
};

Optional: CI integration

Upload lcov.info to Codecov or Coveralls in the GitHub Actions workflow for per-PR coverage diff reporting.

---

Notes

• The lcov reporter produces no human-readable output on its own — pair it with spec or tap using --test-reporter twice
• node_modules/ and core modules are excluded from coverage by default
• Test files themselves are excluded from the coverage report by default; use --test-coverage-exclude to be explicit