JavaScript test coverage made simple. 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. First-class support of ES6/ES2015+ using babel-plugin-Istanbul. Support for the most popular JavaScript testing frameworks. Support for instrumenting subprocesses, using the nyc command-line interface. Adding coverage to your mocha tests could not be easier. Now, simply place the command nyc in front of your existing test command. nyc's instrument command can be used to instrument source files outside of the context of your unit tests. nyc is able to show you all Node processes that are spawned when running a test script under it. By default, nyc uses Istanbul's text reporter. However, you may specify an alternative reporter.