Just a quick explanation of how tools instrument your JavaScript code to record and report code coverage. Common tools:

• istanbul
• nyc with ava are great for coverage of node-running tests (use jsdom to mock a DOM like I do).
• babel-plugin-__coverage__
• karma-coverage with karma and the babel plugin are great for coverage of code when you want to run your tests in a browser
• isparta (no longer maintained, try the babel plugin)

You may also be interested in this video + this repo where I go through setting up code coverage with ES6, Webpack, and Karma. Just replace where I use isparta with using babel-plugin-__coverage__ (like this) and it should work great 👍

I hope this is helpful!

See you on twitter!