Code Quality Rank: L4
Programming language: JavaScript
License: ISC License
Tags: Testing     Coverage     Subprocess     Reporter    
Latest version: v15.1.0

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

  • applications that spawn subprocesses.
  • source mapped coverage of Babel and TypeScript projects

How Istanbul works

Istanbul instruments your ES5 and ES2015+ JavaScript code with line counters, so that you can track how well your unit-tests exercise your codebase.

The nyc command-line-client for Istanbul works well with most JavaScript testing frameworks: tap, mocha, AVA, etc.

Installation & Usage

Use your package manager to add it as a dev dependency: npm i -D nyc or yarn add -D nyc. You can use nyc to call npm scripts (assuming they don't already have nyc executed in them), like so (replace mocha with your test runner everywhere you see it):

  "scripts": {
    "test": "mocha",
    "coverage": "nyc npm run test"

You can use also npx instead of installing nyc as a dependency, but you might get updates you are not ready for; to get around this, pin to a specific major version by specifying, e.g. nyc@14.

  "scripts": {
    "test": "npx nyc@latest mocha"

This is a good way of testing upcoming releases of nyc, usually on the next tag.

Note: If you use jest or tap, you do not need to install nyc. Those runners already have the IstanbulJS libraries to provide coverage for you. Follow their documentation to enable and configure coverage reporting.

Configuring nyc

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

Configuration arguments on the command-line should be provided prior to the program that nyc is executing. As an example, the following command executes ava, and indicates to nyc that it should output both an lcov (lcov.info + html report) and a text-summary coverage report.

nyc --reporter=lcov --reporter=text-summary ava

Babel projects

Please start with the pre-configured @istanbuljs/nyc-config-babel preset. You can add your custom configuration options as shown below.

TypeScript projects

Please start with the pre-configured @istanbuljs/nyc-config-typescript preset.

Adding your overrides

nyc allows you to inherit other configurations using the key extends in the package.json stanza, .nycrc, or YAML files. You can then add the specific configuration options you want that aren't in that particular shared config, e.g.

  "extends": "@istanbuljs/nyc-config-typescript",
  "all": true,
  "check-coverage": true

Configuration files

Any configuration options that can be set via the command line can also be specified in the nyc stanza of your package.json, or within a separate configuration file - a variety of flavors are available:

File name File Association
.nycrc JSON
.nycrc.json JSON
.nycrc.yaml YAML
.nycrc.yml YAML
nyc.config.js CommonJS export

Common Configuration Options

See nyc --help for all options available. You can set these in any of the files listed above, or from the command line. This table is a quick TLDR for the rest of this readme and there are more advanced docs available.

Option name Description Type Default
all Whether or not to instrument all files (not just the ones touched by your test suite) Boolean false
check-coverage Check whether coverage is within thresholds, fail if not Boolean false
extension List of extensions that nyc should attempt to handle in addition to .js Array<String> ['.js', '.cjs', '.mjs', '.ts', '.tsx', '.jsx']
include See selecting files for coverage for more info Array<String> ['**']
exclude See selecting files for coverage for more info Array<String> list
reporter May be set to a built-in coverage reporter or an npm package (dev)dependency Array<String> ['text']
report-dir Where to put the coverage report files String ./coverage
skip-full Don't show files with 100% statement, branch, and function coverage Boolean false
temp-dir Directory to output raw coverage information to String ./.nyc_output

Configuration can also be provided by nyc.config.js if programmed logic is required:

'use strict';

const defaultExclude = require('@istanbuljs/schema/default-exclude');
const isWindows = require('is-windows');

let platformExclude = [
  isWindows() ? 'lib/posix.js' : 'lib/win32.js'

module.exports = {
  exclude: platformExclude.concat(defaultExclude)

Publish and reuse your nyc configuration(s)

To publish and reuse your own nyc configuration, simply create an npm module that exports your JSON config (via index.json or a CJS index.js).

A more advanced use case would be to combine multiple shared configs in a nyc.config.js file:

'use strict';

const babelConfig = require('@istanbuljs/nyc-config-babel');
const hookRunInThisContextConfig = require('@istanbuljs/nyc-config-hook-run-in-this-context');

module.exports = {
  all: true,
  'check-coverage': true

Selecting files for coverage

By default, nyc only collects coverage for source files that are visited during a test. It does this by watching for files that are require()'d during the test. When a file is require()'d, nyc creates and returns an instrumented version of the source, rather than the original. Only source files that are visited during a test will appear in the coverage report and contribute to coverage statistics.

nyc will instrument all files if the --all flag is set or if running nyc instrument. In this case all files will appear in the coverage report and contribute to coverage statistics.

nyc will only collect coverage for files that are located under cwd, and then only files with extensions listed in the extension array.

You can reduce the set of instrumented files by adding include and exclude filter arrays to your config. These allow you to shape the set of instrumented files by specifying glob patterns that can filter files from the default instrumented set. The exclude array may also use exclude negated glob patterns, these are specified with a ! prefix, and can restore sub-paths of excluded paths.

Globs are matched using minimatch.

We use the following process to remove files from consideration:

  1. Limit the set of instrumented files to those files in paths listed in the include array.
  2. Remove any files that are found in the exclude array.
  3. Restore any exclude negated files that have been excluded in step 2.

Using include and exclude arrays

If there are paths specified in the include array, then the set of instrumented files will be limited to eligible files found in those paths. If the include array is left undefined all eligible files will be included, equivalent to setting include: ['**']. Multiple include globs can be specified on the command line, each must follow a --include, -n switch.

If there are paths specified in the exclude array, then the set of instrumented files will not feature eligible files found in those paths. You can also specify negated paths in the exclude array, by prefixing them with a !. Negated paths can restore paths that have been already been excluded in the exclude array. Multiple exclude globs can be specified on the command line, each must follow a --exclude, -x switch.

The default exclude list is defined in the @istanbuljs/schema module. Specifying your own exclude property completely replaces these defaults.

For example, the following nyc config will collect coverage for every file in the src directory regardless of whether it is require()'d in a test. It will also exclude any files with the extension .spec.js.

  "all": true,
  "include": [
  "exclude": [

Note: Be wary of automatic OS glob expansion when specifying include/exclude globs with the CLI. To prevent this, wrap each glob in single quotes.

Including files within node_modules

We always add **/node_modules/** to the exclude list, even if not specified in the config. You can override this by setting --exclude-node-modules=false.

For example, "excludeNodeModules: false" in the following nyc config will prevent node_modules from being added to the exclude rules. The set of include rules then restrict nyc to only consider instrumenting files found under the lib/ and node_modules/@my-org/ directories. The exclude rules then prevent nyc instrumenting anything in a test folder and the file node_modules/@my-org/something/unwanted.js.

  "all": true,
  "include": [
  "exclude": [
  "excludeNodeModules": false

Setting the project root directory

nyc runs a lot of file system operations relative to the project root directory. During startup nyc will look for the default project root directory. The default project root directory is the first directory found that contains a package.json file when searching from the current working directory up. If nyc fails to find a directory containing a package.json file, it will use the current working directory as the default project root directory. You can change the project root directory with the --cwd option.

nyc uses the project root directory when:

  • looking for source files to instrument
  • creating globs for include and exclude rules during file selection
  • loading custom require hooks from the require array

nyc may create artifact directories within the project root, with these defaults:

  • the report directory, <project-root>/coverage
  • the cache directory, <project-root>/node_modules/.cache/nyc
  • the temp directory, <project-root>/.nyc_output

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 esm mocha

Interaction with --all flag

The --require flag also operates on the main nyc process for use by --all. For example, in situations with nyc --all --instrument false and babel-plugin-istanbul setup the --all option only works if --require @babel/register is passed to nyc. Passing it to mocha would cause the tests to be instrumented but unloaded sources would not be seen. The @istanbuljs/nyc-config-babel package handles this for you!


nyc's default behavior is to cache instrumented files to disk to prevent instrumenting source files multiple times, and speed nyc execution times. You can disable this behavior by running nyc with the --cache false flag. You can also change the default cache directory from ./node_modules/.cache/nyc by setting the --cache-dir flag.

Coverage thresholds

You can set custom coverage thresholds that will fail if check-coverage is set to true and your coverage drops below those thresholds. For example, in the following nyc configuration, dropping below 80% branch, line, functions, or statements coverage would fail the build (you can have any combination of these):

  "branches": 80,
  "lines": 80,
  "functions": 80,
  "statements": 80

To do this check on a per-file basis (as opposed to in aggregate), set the per-file option to true.

High and low watermarks

Several of the coverage reporters supported by nyc display special information for high and low watermarks:

  • high-watermarks represent healthy test coverage (in many reports this is represented with green highlighting).
  • low-watermarks represent sub-optimal coverage levels (in many reports this is represented with red highlighting).

You can specify custom high and low watermarks in nyc's configuration:

  "watermarks": {
    "lines": [80, 95],
    "functions": [80, 95],
    "branches": [80, 95],
    "statements": [80, 95]

Parsing Hints (Ignoring Lines)

There may be some sections of your codebase that you wish to purposefully exclude from coverage tracking, to do so you can use the following parsing hints:

  • /* istanbul ignore if */: ignore the next if statement.
  • /* istanbul ignore else */: ignore the else portion of an if statement.
  • /* istanbul ignore next */: ignore the next thing in the source-code ( functions, if statements, classes, you name it).
  • /* istanbul ignore file */: ignore an entire source-file (this should be placed at the top of the file).

Ignoring Methods

You can ignore every instance of a method simply by adding its name to the ignore-class-method array in your nyc config.

  "ignore-class-method": ["render"]

Combining reports from multiple runs

If for whatever reason you have different test runners in your project or a different series of test runs for different kinds of tests, nyc will automatically combine the coverage report for you if configured correctly with the --no-clean flag and the report command. Originally inspired by @janiukjf in #1001, here's an example, where the test:* scripts (not shown) invoke only your test runner(s) and not nyc:

  "scripts": {
    "cover": "npm run cover:unit && npm run cover:integration && npm run cover:report",
    "cover:unit": "nyc --silent npm run test:unit",
    "cover:integration": "nyc --silent --no-clean npm run test:integration",
    "cover:report": "nyc report --reporter=lcov --reporter=text"

What about nyc merge?

The nyc merge command is for producing one raw coverage output file that combines the results from many test runs. So if you had the above setup and needed to produce a single coverage.json for some external tool, you could do:

  "scripts": {
    "cover:merge": "npm run cover:unit && npm run cover:integration && nyc merge .nyc_output coverage.json"

Source-Map support for pre-instrumented codebases

If you opt to pre-instrument your source-code (rather than using a just-in-time transpiler like @babel/register) nyc supports both inline source-maps and .map files.

Important: If you are using nyc with a project that pre-instruments its code, run nyc with the configuration option --exclude-after-remap set to false. Otherwise nyc's reports will exclude any files that source-maps remap to folders covered under exclude rules.

[Integrating with coveralls](./docs/setup-coveralls.md)

[Integrating with codecov](./docs/setup-codecov.md)

[Producing instrumented source](./docs/instrument.md)

Integrating with TAP formatters

Many testing frameworks (Mocha, Tape, Tap, etc.) can produce TAP output. tap-nyc is a TAP formatter designed to look nice with nyc.

Tutorials and Advanced Documentation

See more nyc tutorials and advanced nyc documentation.

Please feel free to contribute documentation to help us improve.

nyc for enterprise

Available as part of the Tidelift Subscription.

The maintainers of nyc and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source dependencies you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact dependencies you use. Learn more.