How to get all selected values from <select multiple=multiple>?

Answer 1

Something like the following would be my choice:

let selectElement = document.getElementById('categorySelect');
let selectedOptions = selectedElement.selectedOptions || [].filter.call(selectedElement.options, option => option.selected);
let selectedValues = [].map.call(selectedOptions, option => option.value);

It's short, it's fast on modern browsers, and we don't care whether it's fast or not on 1% market share browsers.

Note, selectedOptions has wonky behavior on some browsers from around 5 years ago, so a user agent sniff isn't totally out of line here.