codex-lv3-may-2025

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Level 42: Add Test Coverage

Keeping an eye on test coverage helps you understand which parts of your code are exercised by your suite. Let’s install the coverage peer dependency and add a script that makes running coverage painless.

Step 1: Install the coverage reporter

Vitest’s coverage command relies on the V8/Istanbul integration shipped in @vitest/coverage-v8. Install it as a dev dependency using npm:

npm install -D @vitest/coverage-v8

Step 2: Add the coverage script

Update your package.json scripts section so it includes a test:coverage command:

"scripts": {
  "test": "vitest",
  "test:coverage": "vitest run --coverage"
}

Tip: If you already have other scripts (like "dev" or "lint"), just add this line alongside them—keep the trailing commas consistent with the existing JSON.

Step 3: Run the coverage report

npm run test:coverage

Vitest will execute the full suite once and print a table showing statements, branches, functions, and lines covered. The report also lands in the coverage/ directory if you want to inspect the HTML output.

Example console output:

 % Coverage report from v8

----------|---------|----------|---------|---------|-------------------
File      | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s
----------|---------|----------|---------|---------|-------------------
All files |      50 |      100 |      50 |      50 |
 utils.js |      50 |      100 |      50 |      50 | 2
----------|---------|----------|---------|---------|-------------------

Step 4: Review your results

Look for rows that fall below your target percentage and add tests to improve them
Rerun npm run test:coverage after writing new tests to confirm the numbers move in the right direction

Try it: Install the reporter, add the script, run coverage, and note any functions that still need tests before moving on.