I was unhappy with the various contorted examples I found on the Intenet that attempt to do this. Far too fancy by half, and way too enamored with using advanced JS6 constructs for their own sake.
So I wrote my own: the translate function.
It is is super fast, super efficient, very easy to understand, and is the holy grail of what I have been looking for since I started this project a few weeks or so ago: the ability to click on any part of the XVG surface (ie, the SVG area where the designer can do his or her work), and get an exact X/Y coordinate back, no matter what the display screen size or applied transfor might be.
You can see it in action here.
ps: if you have any trouble following my latest pen, here is an excellent, no nonsense no glib, fancy talk explanation of JS event listeners. I wrote my function after watching the first 5 minutes of this.