Top z-index

HTML

<div id="layer-1">layer-1</div>
<div id="layer-2">layer-2</div>
<div id="layer-3">layer-3</div>
<div id="layer-4">layer-4</div>

CSS

#layer-1 { z-index: 1 }
#layer-2 { z-index: 67 }
#layer-3 { z-index: 3 }
#layer-4 { z-index: 4 }

JavaScript

$(document).ready(function() {
    var array = [];
    $("div").each(function() {
        array.push($(this).css("z-index"));
    });
    var index_highest = Math.max.apply(Math, array);
    alert(Integer.valueOf(index_highest));
});