JSFiddle - React, Tailwind, and code Playground

HTML

<div id="computed">This div has width and height set in CSS.</div>

CSS

#computed{
    width: 14em;
    height: 14em;
    background: rgba(00,00,00,.3);
}

JavaScript

$('#computed').click(function(){
    alert($(this).css('width') + "\n" + $(this).width()
         + "\n" + $(this).emWidth());
});
(function($){
    $.fn.emWidth = function(){
        var wpx = this.width();
        var temp = $('<div />').css({
            width:'1em', 
            position: 'absolute'
        });
        this.append(temp);
        var oneEm = temp.width();
        temp.remove();
        var value = wpx/oneEm;
        return Math.round(value*100)/100;
    };
})(jQuery);