SO - 30071009

by Arun P Johny

HTML

<div class="isotope-item 1,2,5,21">...content...</div>

JavaScript

var els = document.getElementsByClassName('isotope-item'),
    el, array;
for (var i = 0; i < els.length; i++) {
    el = els[i];
    el.className = el.className.replace(/(\d+)(,|$|\s)/g, function (all, value) {
        return 'cat' + value + ' '
    })
}