nyc

What is nyc?

The nyc npm package is a command-line tool that allows for JavaScript code coverage. It is built on top of Istanbul and works well with subprocesses. It is often used for tracking how well your unit-tests exercise your codebase.

What are nyc's main functionalities?

Code Coverage

This feature allows you to collect code coverage information from your tests. You can use it with a test runner like mocha by adding it to your npm scripts in your package.json file.

"scripts": {
  "test": "nyc mocha"
}

Check Coverage Thresholds

nyc can enforce coverage thresholds. If code coverage falls below the specified thresholds, nyc will exit with a failure status. This is useful for maintaining a high standard of test coverage in a project.

"scripts": {
  "test": "nyc --check-coverage --lines 95 --functions 95 --branches 95 mocha"
}

Report Generation

After running your tests with nyc, you can generate various types of coverage reports. This example shows how to generate a report in the 'lcov' format, which can be used with tools that support lcov coverage reports.

"scripts": {
  "coverage": "nyc report --reporter=text-lcov > coverage.lcov"
}

Integration with CI/CD

nyc can be integrated with continuous integration/continuous deployment (CI/CD) systems. In this example, coverage information is piped to the 'coveralls' service to track coverage over time.

"scripts": {
  "coveralls": "nyc report --reporter=text-lcov | coveralls"
}

Other packages similar to nyc

istanbul

Istanbul is the underlying tool that nyc is built upon. It provides a JavaScript code coverage tool that computes statement, line, function, and branch coverage with module loader hooks to instrument code on the fly.

c8

c8 is a code coverage tool that uses Node.js' built-in V8 coverage. It is similar to nyc but does not use instrumented code and is built directly on V8's native coverage features, potentially providing more accurate coverage metrics.

jest

Jest is a delightful JavaScript Testing Framework with a focus on simplicity. It works out of the box for any React project. Jest provides its own way to track code coverage without needing an additional package like nyc.

blanket

Blanket is a simple code coverage library for JavaScript that works both in the browser and with Node.js. It is less commonly used now and has been largely superseded by tools like nyc and istanbul.

README

nyc

Istanbul's state of the art command line interface, with support for:

• applications that spawn subprocesses.
• ES2015 transforms, via babel-plugin-istanbul, or source-maps.

Instrumenting your code

You can install nyc as a development dependency and add it to the test stanza in your package.json.

npm i nyc --save-dev

{
  "script": {
    "test": "nyc tap ./test/*.js"
  }
}

Alternatively, you can install nyc globally and use it to execute npm test:

npm i nyc -g

nyc npm test

nyc accepts a wide variety of configuration arguments, run nyc --help for thorough documentation.

Configuration arguments should be provided prior to the program that nyc is executing. As an example, the following command executes npm test, and indicates to nyc that it should output both an lcov and a text-lcov coverage report.

nyc --reporter=lcov --reporter=text-lcov npm test

Support for custom require hooks (babel, webpack, etc.)

nyc supports custom require hooks like babel-register. nyc can load the hooks for you, using the --require flag.

Source maps are used to map coverage information back to the appropriate lines of the pre-transpiled code. You'll have to configure your custom require hook to inline the source map in the transpiled code. For Babel that means setting the sourceMaps option to inline.

Use with babel-plugin-istanbul for ES6/ES7/ES2015 Support

babel-plugin-istanbul can be used to enable better first-class ES6 support.

1. enable the babel-plugin-istanbul plugin:

  {
    "babel": {
      "presets": ["es2015"],
      "env": {
        "test": {
          "plugins": ["istanbul"]
        }
      }
    }
  }

Note: With this configuration, the Istanbul instrumentation will only be active when NODE_ENV or BABEL_ENV is test.

We recommend using the cross-env package to set these environment variables in your package.json scripts in a way that works cross-platform.

1. disable nyc's instrumentation and source-maps:

{
  "nyc": {
    "include": [
      "src/*.js"
    ],
    "require": [
      "babel-register"
    ],
    "sourceMap": false,
    "instrument": false
  }
}

That's all there is to it, better ES6 syntax highlighting awaits:

Support for alternate file extensions (.jsx, .es6)

Supporting file extensions can be configured through either the configuration arguments or with the nyc config section in package.json.

nyc --extension .jsx --extension .es6 npm test

{
  "nyc": {
    "extension": [
      ".jsx",
      ".es6"
    ]
  }
}

Checking coverage

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

nyc check-coverage --lines 95 --functions 95 --branches 95

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

nyc --check-coverage --lines 100 npm test

The above check fails if coverage falls below 100%.

Running reports

Once you've run your tests with nyc, simply run:

nyc report

To view your coverage report:

you can use any reporters that are supported by istanbul:

nyc report --reporter=lcov

Excluding files

You can tell nyc to exclude specific files and directories by adding an nyc.exclude array to your package.json. Each element of the array is a glob pattern indicating which paths should be omitted.

Globs are matched using micromatch.

In addition to patterns specified in the package, nyc will always exclude files in node_modules.

For example, the following config will exclude everything in node_modules, any files with the extension .spec.js, and anything in the build directory:

{
  "nyc": {
    "exclude": [
      "**/*.spec.js",
      "build"
    ]
  }
}

Note: exclude defaults to ['test', 'test{,-*}.js', '**/*.test.js', '**/__tests__/**'], which would exclude test/__tests__ directories as well as test.js, *.test.js, and test-*.js files. Specifying your own exclude property overrides these defaults.

Including files

As an alternative to providing a list of files to exclude, you can provide an include key to specify specific files that should be covered:

{
  "nyc": {
    "include": ["**/build/umd/moment.js"]
  }
}

Note: include defaults to ['**']

Include reports for files that are not required

By default nyc does not collect coverage for files that have not been required, run nyc with the flag --all to enable this.

Require additional modules

The --require flag can be provided to nyc to indicate that additional modules should be required in the subprocess collecting coverage:

nyc --require babel-core/register --require babel-polyfill mocha

Caching

You can run nyc with the optional --cache flag, to prevent it from instrumenting the same files multiple times. This can significantly improve runtime performance.

Configuring nyc

Any configuration options that can be set via the command line can also be specified in the nyc stanza of your package.json (these will not affect nyc subcommands):

{
  "description": "These are just examples for demonstration, nothing prescriptive",
  "nyc": {
    "lines": 99,
    "statements": 99,
    "functions": 99,
    "branches": 99,
    "include": [
      "src/**/*.js"
    ],
    "exclude": [
      "src/**/*.spec.js"
    ],
    "reporter": [
      "lcov",
      "text-summary"
    ],
    "require": [
      "./test/helpers/some-helper.js"
    ],
    "extension": [
      ".jsx"
    ],
    "cache": true,
    "all": true,
    "check-coverage": true,
    "report-dir": "./alternative"
  }
}

Instrumenting source files

nyc's instrument command can be used to instrument source files outside of the context of your unit-tests:

instrument the entire ./lib folder:

nyc instrument ./lib ./output

Integrating with coveralls

coveralls.io is a great tool for adding coverage reports to your GitHub project. Here's how to get nyc integrated with coveralls and travis-ci.org:

1. add the coveralls and nyc dependencies to your module:

npm install coveralls nyc --save

1. update the scripts in your package.json to include these bins:

{
   "script": {
     "test": "nyc tap ./test/*.js",
     "coverage": "nyc report --reporter=text-lcov | coveralls"
   }
}

1. For private repos, add the environment variable COVERALLS_REPO_TOKEN to travis.

2. add the following to your .travis.yml:

after_success: npm run coverage

That's all there is to it!

Note: by default coveralls.io adds comments to pull-requests on GitHub, this can feel intrusive. To disable this, click on your repo on coveralls.io and uncheck LEAVE COMMENTS?.

Integrating with codecov

nyc npm test && nyc report --reporter=text-lcov > coverage.lcov && codecov

codecov is a great tool for adding coverage reports to your GitHub project, even viewing them inline on GitHub with a browser extension:

Here's how to get nyc integrated with codecov and travis-ci.org:

1. add the codecov and nyc dependencies to your module:

npm install codecov nyc --save-dev

1. update the scripts in your package.json to include these bins:

{
   "script": {
     "test": "nyc tap ./test/*.js",
     "coverage": "nyc report --reporter=text-lcov > coverage.lcov && codecov"
   }
}

1. For private repos, add the environment variable CODECOV_TOKEN to travis.

2. add the following to your .travis.yml:

after_success: npm run coverage

That's all there is to it!

Changelog

7.1.0 (2016-07-24)

Bug Fixes

• make --all flag work with files with extensions other than .js (#326) (d0a8674)
• work around for Windows path issue nodejs/node#6624 (6b1fed0)

Features

• nyc no longer tries to run arguments passed to the instrumented bin (#322) (e0a8c0b)
• use istanbul-lib-hook to wrap require and support vm hooks (#308) (2b64cf8)

<a name="7.0.0"></a>