Yet another JS code coverage tool that computes statement, line, function and branch coverage with module loader hooks to transparently add coverage when running tests. Supports all JS coverage use cases including unit tests, server side functional tests

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Istanbul - a JS code coverage tool written in JS

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  • All-javascript instrumentation library that tracks statement, branch,
and function coverage.
  • Module loader hooks to instrument code on the fly
  • Command line tools to run node unit tests "with coverage turned on" and no cooperation
whatsoever from the test runner
  • Multiple report formats: HTML, LCOV, Cobertura and more.
  • Ability to use as middleware when serving JS files that need to be tested on the browser.
  • Can be used on the command line as well as a library
  • Based on the awesome esprima parser and the equally awesome escodegen code generator
  • Well-tested on node (prev, current and next versions) and the browser (instrumentation library only)

Use cases

Supports the following use cases and more
  • transparent coverage of nodejs unit tests
  • instrumentation/ reporting of files in batch mode for browser tests
  • Server side code coverage for nodejs by embedding it as custom middleware

Getting started

$ npm install -g istanbul
The best way to see it in action is to run node unit tests. Say you have a test script test.js that runs all tests for your node project without coverage.
$ cd /path/to/your/source/root
$ istanbul cover test.js
and this should produce a coverage.json, and lcov-report/*html under ./coverage
Sample of code coverage reports produced by this tool (for this tool!):
HTML reports


Drop a .istanbul.yml file at the top of the source tree to configure istanbul. istanbul help config tells you more about the config file format.

The command line

$ istanbul help
gives you detailed help on all commands.
Usage: istanbul help config | <command>

`config` provides help with istanbul configuration

Available commands are:

              checks overall/per-file coverage against thresholds from coverage
              JSON files. Exits 1 if thresholds are not met, 0 otherwise

      cover   transparently adds coverage information to a node command. Saves
              coverage.json and reports at the end of execution

      help    shows help

              instruments a file or a directory tree and writes the
              instrumented code to the desired output location

      report  writes reports for coverage JSON objects produced in a previous

      test    cover a node command only when npm_config_coverage is set. Use in
              an `npm test` script for conditional coverage

Command names can be abbreviated as long as the abbreviation is unambiguous

To get detailed help for a command and what command-line options it supports, run:
istanbul help <command>
(Most of the command line options are not covered in this document.)

The cover command

$ istanbul cover my-test-script.js -- my test args
# note the -- between the command name and the arguments to be passed
The cover command can be used to get a coverage object and reports for any arbitrary node script. By default, coverage information is written under ./coverage - this can be changed using command-line options.
The cover command can also be passed an optional --handle-sigint flag to enable writing reports when a user triggers a manual SIGINT of the process that is being covered. This can be useful when you are generating coverage for a long lived process.

The test command

The test command has almost the same behavior as the cover command, except that it skips coverage unless the npm_config_coverage environment variable is set.
This command is deprecated since the latest versions of npm do not seem to set the npm_config_coverage variable.

The instrument command

Instruments a single JS file or an entire directory tree and produces an output directory tree with instrumented code. This should not be required for running node unit tests but is useful for tests to be run on the browser.

The report command

Writes reports using coverage*.json files as the source of coverage information. Reports are available in multiple formats and can be individually configured using the istanbul config file. See istanbul help report for more details.

The check-coverage command

Checks the coverage of statements, functions, branches, and lines against the provided thresholds. Positive thresholds are taken to be the minimum percentage required and negative numbers are taken to be the number of uncovered entities allowed.

Ignoring code for coverage

  • Skip an if or else path with /* istanbul ignore if */ or /* istanbul ignore else */ respectively.
  • For all other cases, skip the next 'thing' in the source with: /* istanbul ignore next */

See for the spec.


All the features of istanbul can be accessed as a library.

Instrument code

var istanbul = require('istanbul');
var instrumenter = new istanbul.Instrumenter();

var generatedCode = instrumenter.instrumentSync('function meaningOfLife() { return 42; }',

Generate reports given a bunch of coverage JSON objects

var istanbul = require('istanbul'),
    collector = new istanbul.Collector(),
    reporter = new istanbul.Reporter(),
    sync = false;

collector.add(obj2); //etc.

reporter.addAll([ 'lcov', 'clover' ]);
reporter.write(collector, sync, function () {
    console.log('All reports generated');

For the gory details consult the public API

Multiple Process Usage

Istanbul can be used in a multiple process environment by running each process with Istanbul, writing a unique coverage file for each process, and combining the results when generating reports. The method used to perform this will depend on the process forking API used. For example when using the cluster module you must setup the master to start child processes with Istanbul coverage, disable reporting, and output coverage files that include the PID in the filename. Before each run you may need to clear out the coverage data directory.
if(cluster.isMaster) {
    // setup cluster if running with istanbul coverage
    if(process.env.running_under_istanbul) {
        // use coverage for forked process
        // disabled reporting and output for child process
        // enable pid in child process coverage filename
            exec: './node_modules/.bin/istanbul',
            args: [
                'cover', '--report', 'none', '--print', 'none', '--include-pid',
                process.argv[1], '--'].concat(process.argv.slice(2))
    // ...
    // ... cluster.fork();
    // ...
} else {
    // ... worker code


For details on the format of the coverage.json object, see here.


istanbul is licensed under the BSD License.

Third-party libraries

The following third-party libraries are used by this module:
  • abbrev: - to handle command abbreviations
  • async: - for parallel instrumentation of files
  • escodegen: - for JS code generation
  • esprima: - for JS parsing
  • fileset: - for loading and matching path expressions
  • handlebars: - for report template expansion
  • js-yaml: - for YAML config file load
  • mkdirp: - to create output directories
  • nodeunit: - dev dependency for unit tests
  • nopt: - for option parsing
  • once: - to ensure callbacks are called once
  • resolve: - for resolving a post-require hook module name into its main file.
  • rimraf - - dev dependency for unit tests
  • which: - to resolve a node command to a file for the cover command
  • wordwrap: - for prettier help
  • prettify: - for syntax colored HTML reports. Files checked in under lib/vendor/

Inspired by

  • YUI test coverage - - the grand-daddy of JS coverage tools. Istanbul has been specifically designed to offer an alternative to this library with an easy migration path.
  • cover: - the inspiration for the cover command, modeled after the run command in that tool. The coverage methodology used by istanbul is quite different, however

Shout out to

* [mfncooper]( - for great brainstorming discussions
* [reid](, [davglass](, the YUI dudes, for interesting conversations, encouragement, support and gentle pressure to get it done :)

Why the funky name?

Since all the good ones are taken. Comes from the loose association of ideas across coverage, carpet-area coverage, the country that makes good carpets and so on...