The Node and browser environments provide different functions, in addition to the process (Node-only) and window (browser-only) objects. If you want an NPM package to work on both, there's a way to specify different source files in the package.json file.

Say you have a package in a private Bitbucket repo that you want to include as a dependency. It turns out you don't have to publish it to the npm registry and trust that it's "private", like they recommend, and you also don't have to create your own registry, which looks even more painful.