NodeJS - Getting test coverage from child processes

This is a problem that I keep running into. I like to use a combination of Grunt, Mocha and Blanket to get coverage reports from my unit tests. The problem is that this creates a global variable to collect the coverage data in the process in which the code under test runs. Normally this is the same process in which Grunt and Mocha are running, however I have a common class of tests in which this is not true.

When I want to perform an end to end integration test or test an entry point to a command line tool then usually I want to use child_process.exec or child_process.spawn to kick it off. This first became apparent when I wanted to test a plugin for Grunt itself, grunt-mocha-test. The workaround was to create a special script that could be launched in place of Grunt that would programatically call Grunt, collect the coverage data and then write it to a file. This script would then be launched by a wrapper to child_process.exec that would collect the coverage data after the exec had completed and merge it with the coverage data from the parent process.

This works well but is not very portable. The challenge then is to create a generic solution to the problem and here it is.

The newly published cover-child-process module can merge coverage data from Blanket instrumented source files running in child processes with the coverage data collected in the parent process.

Support for source instrumented with other coverage tools should be easy to add in the future too. Yay :)