Mocha test coverage

The latest release of Mocha adds two new reporters, now totaling 13 packaged with the library. The two new ones are “json-cov” and “html-cov”, the latter inherits from the former in order to produce a single-page test coverage report.

If you’re curious what a live example looks like, check out this page.

Setup

With Node’s require.paths array out of the picture there’s a bit of boiler-plate involved in setting up test coverage for your project, but it’s pretty minimal. First you’ll want to install the jscoverage executable, this program parses your source code and spits out an instrumented version, effectively a bunch of lines that look like this:

  $_jscoverage[filename][line]++;

This enables libraries like Mocha to execute the code as they normally would, however we can then extract this coverage information and generate fancy reports. jscoverage takes the input directory, and an output directory, I usually calls this “lib-cov”:

 $ jscoverage lib lib-cov

Typically when testing your library you’ll use relative require() calls that look like this: var express = require('../'), and the library entry point is ./index.js. Normally this file might look something like below, or you may not have an ./index.js file at all.

  module.exports = require('./lib/express');

By using ./index.js we can then alter this statement to conditionally export the instrumented version when the EXPRESS_COV environment variable is present:

 module.exports = process.env.EXPRESS_COV
   ? require('./lib-cov/express')
   : require('./lib/express');

Then all you need to do is invoke mocha with --reporter html-cov and redirect the stdout to a file such as coverage.html. Line 25 the test-cov target of the Express Makefile illustrates how I typically handle this but depending on your setup it may vary.

That’s it! remember to add “coverage.html” to your .npmignore and .gitignore files ;)