npm module code coverage in 2 steps

npm module code coverage in 2 steps

We recently discovered the code coverage tool Istanbul, and were pleased to find that some of our small and well tested modules had 100% coverage. However, some modules had un-tested statements, lines, functions or branches. This made me mad! I set about adding coverage to my modules.

Following these two simple steps you can quickly and simply add code coverage reporting to your existing unit tests:

1. Install Istanbul as a dev dependency

You'll need to add Istanbul to your dev dependency list (not dependencies, because you don't want your consumers to have to download it when you install your module). You could manually edit your package.json, then npm install it (like a chump), or you could install it and update your package.json in one fell swoop:

npm install istanbul --save-dev

2. Amend your npm test script

Any self respecting module author lists their test command in their package.json. If you don't, you should feel ashamed and add it right now.

I'll take you through my new command:

"scripts": {
    "./node_modules/.bin/istanbul test ./node_modules/.bin/_mocha -- -R spec"

node_modules/.bin/istanbul test runs the istanbul executable with the 'test' command.

node_modules/.bin/_mocha points to the mocha executable which is how my tests are run.

-R spec are my arguments for mocha.

-- tells istanbul to pass the following arguments the test process rather than consume them itself.

Mocha users, note the _mocha executable path contains an underscore. This is because the executable without an underscore forks processes to run your tests, but istanbul needs the tests in the same process. Other test framework usage may vary.

You can now run your tests with:

npm test

Istanbul just runs your tests for you without instrumenting.

And generate a coverage report with:

npm test --coverage

You will get some terminal output, with any luck 100% coverage like my module:

===== Coverage summary ======
Statements : 100% ( 64/64 )
Branches : 100% ( 30/30 )
Functions : 100% ( 13/13 )
Lines : 100% ( 58/58 )

…and an html coverage report at ./coverage

Optional extras:

If (like me) you don't want your coverage report cluttering up your repo, add coverage to your .gitignore

That's it. Happy coveraging!

Want to discuss a project?

Got a question? Want some more detail? We'd love to hear from you, contact Jason on +44 (0)1923 261166 or


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