nyc

by istanbuljs

istanbuljs / nyc

the Istanbul command line interface

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nyc

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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.
[email protected]
.
{
  "scripts": {
    "test": "npx [email protected] 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
|
['.js', '.cjs', '.mjs', '.ts', '.tsx', '.jsx']
| |
include
| See selecting files for coverage for more info |
Array
|
['**']
| |
exclude
| See selecting files for coverage for more info |
Array
| list | |
reporter
| May be set to a built-in coverage reporter or an npm package (dev)dependency |
Array
|
['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 = { ...babelConfig, ...hookRunInThisContextConfig, 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": [
    "src/**/*.js"
  ],
  "exclude": [
    "**/*.spec.js"
  ]
}

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": [
    "lib/**",
    "node_modules/@my-org/**"
  ],
  "exclude": [
    "node_modules/@my-org/something/unwanted.js",
    "**/test/**"
  ],
  "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,
    /coverage
  • the cache directory,
    /node_modules/.cache/nyc
  • the temp directory,
    /.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!

Caching

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

Integrating with codecov

Producing instrumented source

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.

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