Return the highest z-Index used across all elements

getHighIndex.js

/**
 * Return the highest z-Index used across all elements
**/
function getHighIndex (selector) {
    // No granularity by default; look at everything
    if (!selector) { selector = '*' };

    var elements = document.querySelectorAll(selector) ||
                   oXmlDom.documentElement.selectNodes(selector),
        i = 0,
        e, s,
        max = elements.length,
        found = [];

    for (; i < max; i += 1) {
        e = window.getComputedStyle(elements[i], null).zIndex || elements[i].currentStyle.zIndex;
        s = window.getComputedStyle(elements[i], null).position || elements[i].currentStyle.position;
        
        // Statically positioned elements are not affected by zIndex
        if (e && s !== "static") {
          found.push(parseInt(e, 10));
        }
    }

    return found.length ? Math.max.apply(null, found) : 0;
}

You could write:

if (!selector) { selector = '*' };

more modernly as:

selector || (selector = '*');

Not better. Not worse. Cooler.

Totally missed this comment! I chose the former for clarity, but yes there is a cooler way. ;-)

Not sure why but it gives me NaN on Firefox's console.