The c8 npm package is a code coverage tool for JavaScript that works with the native V8 coverage functionality built into Node.js. It collects and reports on the test coverage of your code, helping you understand which parts of your codebase are covered by tests and which are not.
Code Coverage Reporting
This command runs your Node.js script with coverage tracking, and upon completion, it outputs a coverage report. The report shows the percentage of code executed by your tests, helping you identify untested parts of your codebase.
npx c8 node script.jsIntegration with Testing Frameworks
You can use c8 in conjunction with testing frameworks like Mocha. This command will run Mocha tests with coverage tracking, and then generate a coverage report.
npx c8 mochaCustom Report Formats
c8 allows you to specify one or more reporters that determine the format of the coverage report. In this example, it generates both 'lcov' and 'text' format reports.
npx c8 --reporter=lcov --reporter=text node script.jsCheck Coverage Thresholds
This feature allows you to enforce coverage thresholds. If the coverage falls below the specified percentages for lines, functions, or branches, c8 will exit with a non-zero status, which is useful for CI/CD pipelines.
npx c8 --check-coverage --lines 95 --functions 95 --branches 95 node script.jsPreviously known as Istanbul, nyc is another popular code coverage tool for JavaScript. It works with a wide range of testing frameworks and can instrument code for coverage reporting. Compared to c8, nyc has been around longer and has more configuration options, but c8 is built to leverage the native V8 coverage and might be faster and more accurate for Node.js applications.
Jest is a testing framework that includes built-in code coverage reporting. While Jest is primarily a testing framework, its coverage tool is often compared to standalone tools like c8. Jest's coverage is implemented using Istanbul under the hood. Compared to c8, Jest provides an all-in-one solution for testing and coverage but may not be as lightweight if you only need coverage reporting.
Coveralls is a web service that helps you track your code coverage over time, ensure that all your new code is fully covered, and integrate with your CI environment. While not a coverage tool itself, it works with tools like c8 or nyc to provide visibility into code coverage statistics. Compared to c8, Coveralls adds a layer of historical tracking and visualization to coverage data.
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.
--allBy 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 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.
run c8 report to regenerate reports after c8 has already been run.
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
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 */.
const myVariable = 99
/* c8 ignore next */
if (process.platform === 'win32') console.info('hello world')
const myVariable = 99
/* c8 ignore next 3 */
if (process.platform === 'win32') {
console.info('hello world')
}
/* c8 ignore start */
function dontMindMe() {
// ...
}
/* c8 ignore stop */
const myVariable = 99
const os = process.platform === 'darwin' ? 'OSXy' /* c8 ignore next */ : 'Windowsy'
c8 uses
native V8 coverage,
make sure you're running Node.js >= 10.12.0.
c8See the contributing guide here.
FAQs
output coverage reports using Node.js' built in coverage
The npm package c8 receives a total of 2,076,345 weekly downloads. As such, c8 popularity was classified as popular.
We found that c8 demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 2 open source maintainers collaborating on the project.
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