c8 vs nyc: JavaScript coverage compared

The short answer

Choose c8 when you want to collect coverage from Node’s V8 runtime. Keep nyc when your project already uses Istanbul instrumentation and that setup works well. Neither choice requires changing your test framework.

nyc is Istanbul’s command-line tool. c8 collects V8 coverage and can use Istanbul’s reporters. Similar-looking reports do not mean the collection methods—or the things they count—are identical. See the c8 documentation and nyc documentation.

Run the same suite

For a Node test suite with source files in src:

npm install -D c8
npx c8 --all --include='src/**/*.cjs' node --test

The corresponding nyc command is:

npm install -D nyc
npx nyc --all --include='src/**/*.cjs' node --test

Use your actual source extension and test command. Keep generated files out of the source scope. The --all option includes eligible files that the tests never load; otherwise a high percentage can hide entire untested files.

For TypeScript, check that the report points to your original source, not only the compiled JavaScript. c8 supports source-map remapping. nyc documents a TypeScript configuration preset. Neither command replaces your compiler or test runner.

Why their percentages can differ

We ran both tools against the same small CommonJS fixture and Node test suite. These are the recorded totals:

Both show 100%, but use different denominators. Do not treat a changed percentage after switching tools as proof that your tests improved. Inspect the highlighted source and the missing cases.

Our fixture exercises both outcomes of signedIn && !expired, but never tries a signed-in session that has expired. Neither standard report measures whether each condition independently changes the decision. That is the question MC/DC answers.

What Supercov adds

Supercov is another way to measure the existing suite, with MC/DC, per-test coverage, and source-to-assertion review. It does not replace c8 or nyc’s reporting workflow one-for-one.

Coverage features

Feature	c812.0.0	nyc / Istanbul18.0.0	Supercov0.0.47
Line coverage	Supported	Supported	Supported
Branch coverage	Supported	Supported	Supported
MC/DC	Not built in	Not built in	Supported
Per-test coverage	Not built in	Not built in	Supported
Assertion coverage	Not built in	Not built in	Supported

✓ Supported · × Not built in. Standard coverage workflows in the versions shown; custom plugins are outside this table.

Vitest uses its standard V8 or Istanbul coverage provider here. Supercov works with your existing test runner.

Per-test coverage means individual tests, not just test files. Supercov needs a supported runner. Assertion coverage links source to checked properties through agent review; it is not a mutation score or a correctness guarantee.

npx supercov -- node --test
npx supercov runs latest gaps

If your goal is to find unexecuted code in a familiar coverage report, c8 or nyc may already be enough. If a green report still leaves you asking “which condition or result did we never check?”, use the deeper measurements to investigate.

Using Vitest? Prefer its integrated providers over wrapping it in another collector. See Vitest coverage: V8 vs Istanbul.