You seem to mix two things together. To use javascript modules, you need to use JSBuild which compiles the “required” modules into one single JS file. I am not sure, but “CommonJS” sounds like a very old concept to me (RequireJS?).
Okay, I found my mistake. I’ve tried to address node modules within my theme folder (my JS file is also in themes/theme-name/assets/js/). This does not work. As soon as I put my package.json in the root folder and install the node modules, it works just fine! (And yes, also with _underscore.js). Don’t know what’s the best way to include node dependencies in a theme, but probably that’s another topic. Thanks!