Dear Rossette,

Use Javascript action for things do not have a rendering. 

https://docs.mendix.com/howto/extensibility/best-practices-javascript-actions#2-2-6-using-external-dependencies-in-the-browser

Use widgets for thing that should render

https://docs.mendix.com/howto/extensibility/pluggable-widgets

In pluggable widgets, you can use external library via NPM.

Cheers, Andries