webpack-library-starter | Webpack based boilerplate for producing libraries

It means that you want to process files in "two modes". At first, webpack processes files in dev mode and after that in build mode.

The problem is that Mocha tests are executed with Node.js, meaning by default, there is no browser available that can provide a DOM that provides APIs like document or window.

A popular tool to provide a DOM for these tests is JSDOM.

Here's a simple example how you could test your class MyClass:

This project template is somewhat ill-advised. Webpack creates packages that are meant to be loaded in a browser environment. Mocha, however, runs in node. So you have to take into account what transmogrifications webpack does to the bundled code and what happens when this code is fed back into node.

When babel-polyfill is bundled by webpack, there's this implicit assumption that the bundle will be loaded in a browser and some specific globals (namely, that this in unbound functions refers to window) will be there.

The proper way to load babel-polyfill in node is with node's own require. For example, change the test script in package.json to:

"test": "mocha --compilers js:babel-core/register --colors -r babel-polyfill ./test/*.spec.js"

Your library's bundle should not load babel-polyfill at all though. If your library initializes babel-polyfill and someone includes your library into their project which also initializes babel-polyfill, they'll get an exception stating, quote, "only one instance of babel-polyfill is allowed".