Code coverage is a valuable metric for understanding the quality of your test suite. In this post we will show how to quickly generate code coverage statistics for your JavaScript and Node.js libraries using Mocha, Travis CI, Istanbul, and Coveralls.

Test Suite with Mocha

We need a test suite before we can generate code coverage. Our favorite testing framework is Mocha with Chai for assertions. A test file might look like this:

var expect = require('chai').expect;

describe('Array', function(){
  describe('#indexOf()', function(){
    it('should return -1 when the value is not present', function(){
      var array = [1,2,3];

Then in our package.json file we would add a test script to run with npm test.

"scripts": {
  "test": "mocha"

Automated Testing with Travis CI

We can automatically run our test suite for each commit using Travis CI. Sign up, add your GitHub repository, then add a .travis.yml file to the repo.

language: node_js
- '0.10'
- 'npm test' # tell Travis how to run your test suite

Every time a commit is pushed to your repository on any branch, Travis will automatically queue up a test runner that clones your repository, installs all dependencies, runs your test suite, and reports the status. You can even add a status badge to your repos README that shows the current build status and acts as a handy link for Travis. The best part is it’s 100% free for public GitHub repos.

Code Coverage with Istanbul

Istanbul is the only viable JavaScript code coverage tool we are aware of. It’s fast, easy to use, can be installed via npm, and doesn’t need to be installed globally. To run, just add another entry in the package.json scripts field.

  "coverage": "istanbul cover ./node_modules/mocha/bin/_mocha"

You’ll notice that we have to call the mocha bin file directly. This is because the main mocha file forks into a child process as explained here.

When you run npm run coverage it will generate coverage statistics in the coverage directory including a browsable HTML report in coverage/lcov-report/index.html. We suggest adding coverage to your .gitignore file so that it’s not committed to your repo.

Automated Code Coverage with Coveralls

Similar to how we used Travis CI to automate our testing, we can use Coveralls to automate our code coverage. The first step is to sign up and enable your repository. This will generate a token which you need to add to a .coveralls.yml file.

repo_token: mycoverallsrepoymltoken

Second, you need to install the coveralls node package. This allows us to send our node coverage stats to Coveralls so that they can store and process it.

Third, we need a new command in our scripts field:

  "coveralls": "npm run coverage && cat ./coverage/ | ./node_modules/coveralls/bin/coveralls.js"

This command will run our test suite to gather coverage stats then pipe them into the coveralls node package which sends the stats to coveralls.

The last thing to do is update .travis.yml so that our new coveralls command is run automatically by Travis each time a commit is pushed to our repository. This will still run tests and track our build status while also allowing coveralls to track changes to our code coverage.

- 'npm run coveralls' # tell Travis how to run your test suite

An option step, though one we highly recommend, is to add a coverage badge to your README that shows off your coverage percentage and adds a quick link to Coveralls browsable coverage stats.


Our full scripts field in package.json now looks like this:

"scripts": {
  "test": "mocha",
  "coverage": "istanbul cover ./node_modules/mocha/bin/_mocha",
  "coveralls": "npm run coverage && cat ./coverage/ | ./node_modules/coveralls/bin/coveralls.js"

Of course, you can add additional scripts. We encourage you to browse the documentation for Mocha, Travis CI, Istanbul, and Coveralls to see what else is possible.

See FS-Traversal for a real example of the setup we described here.