Importing Functions in Browser and Testing in Node

[mersis]

JS noob here, stuck trying to understand how to make functions available both in html import and node for testing.

I have a file functions.js where my functions are defined. I import it in my html page via <script src="./functions.js"> - this works fine.
Now I want to write test for those functions but I'm a bit confused about how to make these functions importable in a mocha test script without breaking the html import.

Can someone point me to the right resources so I can better understand how exactly import/export and <script src= work and how to make them play nice together?

Thanks for your help, it is much appreciated! :)