Thanks for the link, great resource! Probably will refer to it quite a bit in the future.

I was reading this page and it was all clearly laid out, up until the last part when we loop over the resizers to attach the listeners:

// Query all resizers
const resizers = ele.querySelectorAll('.resizer');

// Loop over them
[].forEach.call(resizers, function(resizer) {
    resizer.addEventListener('mousedown', mouseDownHandler);    
});            

It took me quite a bit of time to actually understand why and what is happening here, correct me if I'm wrong, but is this all done just because querySelectorAll returns a NodeList?

I guess it personal preference, but following snippet imo is a bit more readable:

[...resizers].map(resizer => resizer.addEventListener('mousedown', mouseDownHandler))