c8 - native V8 code-coverage

Code-coverage using Node.js' built in functionality that's compatible with Istanbul's reporters.

Like nyc, c8 just magically works:

npm i c8 -g
c8 node foo.js

The above example will output coverage metrics for foo.js.

CLI Options / Configuration

c8 can be configured via command-line flags, a c8 section in package.json, or a JSON configuration file on disk.

A configuration file can be specified by passing its path on the command line with --config or -c. If no config option is provided, c8 searches for files named .c8rc, .c8rc.json, .nycrc, or .nycrc.json, starting from cwd and walking up the filesystem tree.

When using package.json configuration or a dedicated configuration file, omit the -- prefix from the long-form of the desired command-line option.

Here is a list of common options. Run c8 --help for the full list and documentation.

Checking for "full" source coverage using --all

By default v8 will only give us coverage for files that were loaded by the engine. If there are source files in your project that are flexed in production but not in your tests, your coverage numbers will not reflect this. For example, if your project's main.js loads a.js and b.js but your unit tests only load a.js your total coverage could show as 100% for a.js when in fact both main.js and b.js are uncovered.

By supplying --all to c8, all files in directories specified with --src (defaults to cwd) that pass the --include and --exclude flag checks, will be loaded into the report. If any of those files remain uncovered they will be factored into the report with a default of 0% coverage.

SourceMap Support

c8 can handle source-maps, for remapping coverage from generated code to original source files (useful for TypeScript, JSX, etc).

Source map files versus inline source maps

Just-in-time instrumented codebases will often insert source maps inline with the .js code they generate at runtime (e.g, @babel/register can be configured to insert a source map footer).

Pre-instrumented codebases, e.g., running tsc to generate .js in a build folder, may generate either inline source maps, or a separate .map file stored on disk.

c8 can handle loading both types of source maps.

Exclude after remap

Depending on the size and configuration of your project, it may be preferable to apply exclusion logic either before or after source-maps are used to remap compiled to original source files.

--exclude-after-remap is used to control this behaviour.

c8 report

run c8 report to regenerate reports after c8 has already been run.

Checking coverage

c8 can fail tests if coverage falls below a threshold. After running your tests with c8, simply run:

c8 check-coverage --lines 95 --functions 95 --branches 95

c8 also accepts a --check-coverage shorthand, which can be used to both run tests and check that coverage falls within the threshold provided:

c8 --check-coverage --lines 100 npm test

The above check fails if coverage falls below 100%.

To check thresholds on a per-file basis run:

c8 check-coverage --lines 95 --per-file

If you want to check for 100% coverage across all dimensions, use --100:

c8 --100 npm test

Is equivalent to

c8 --check-coverage --lines 100 --functions 100 --branches 100 --statements 100  npm test

The --100 flag can be set for the check-coverage as well:

c8 check-coverage --100

Using Monocart coverage reports (experimental)

Monocart is an alternate library for outputting v8 code coverage data as Istanbul reports.

Monocart also provides reporters based directly on v8's byte-offset-based output. Such as, console-details and v8. This removes a complex transformation step and may be less bug prone for some environments.

Example usage:

c8 --experimental-monocart --reporter=v8 --reporter=console-details node foo.js

NOTE: Monocart requires additional monocart-coverage-reports to be installed:

npm i monocart-coverage-reports@2 --save-dev

Ignoring Uncovered Lines, Functions, and Blocks

Sometimes you might find yourself wanting to ignore uncovered portions of your codebase. For example, perhaps you run your tests on Linux, but there's some logic that only executes on Windows.

To ignore lines, blocks, and functions, use the special comment:

/* c8 ignore next */.

Ignoring the next line

const myVariable = 99
/* c8 ignore next */
if (process.platform === 'win32') console.info('hello world')

Ignoring the next N lines

const myVariable = 99
/* c8 ignore next 3 */
if (process.platform === 'win32') {
  console.info('hello world')
}

Ignoring all lines until told

/* c8 ignore start */
function dontMindMe() {
  // ...
}
/* c8 ignore stop */

Ignoring a block on the current line

const myVariable = 99
const os = process.platform === 'darwin' ? 'OSXy' /* c8 ignore next */ : 'Windowsy'

Supported Node.js Versions

c8 uses native V8 coverage, make sure you're running Node.js >= 12.

Contributing to c8

See the contributing guide here.