Coverage

Coverage is a framework which runs your tests, keeping track of every linearly independent paths through your code and marking which path is or isn’t touched by a test. You know your code is 100% tested when every statement, branch (of ifs and switches), functions and line is contemplated.

Istanbul

Istanbul is the instrumentation tool, nyc is its command line client. Simply precede your test command line with nyc and at the end of all tests you’ll get a report line this:

If you wish to generate a neat HTML to visualize your code marked with coverage information, precede your test command with nyc --reporter=html

Codecov and Coveralls

Codecov and Coveralls are, might I say, identical services. They display a report very much like the one you get from nyc --reporter=html. Both read the LCOV format thus requiring you to run nyc report --reporter=text-lcov and if you ask stackshare, both have very similar popularity.