JSFiddle - React, Tailwind, and code Playground

HTML

<div id="foo">hello</div>

CSS

#foo
{
    background: url('images/spacer.gif?bar=411');
}

JavaScript

alert(getCustomCSSProperty('foo', 'bar'));

function getCustomCSSProperty(elId, propName)
{
    var obj = document.getElementById(elId);
    var bi = obj.currentStyle ? obj.currentStyle.backgroundImage : document.defaultView.getComputedStyle(obj, null).getPropertyValue('background-image');
    var biurl = RegExp('url\\(["\\\']?([^"\\\']+)["\\\']?\\)').exec(bi);
    return getParameterByName(propName, biurl[1]);
}

function getParameterByName(name, qs) {
    var match = RegExp('[?&]' + name + '=([^&]*)').exec(qs);
    return match && decodeURIComponent(match[1].replace(/\+/g, ' '));
}