How to find the highest z-index using jQuery

Question

I have a number of div elements with different z-index. And I want to find the highest z-index among these divs - how can I achieve it?

CSS:

#layer-1         


        

Answer 1

Here is a very concise method:

var getMaxZ = function(selector){
    return Math.max.apply(null, $(selector).map(function(){
        var z;
        return isNaN(z = parseInt($(this).css("z-index"), 10)) ? 0 : z;
    }));
};

Usage:

getMaxZ($("#layer-1,#layer-2,#layer-3,#layer-4"));

Or, as a jQuery extension:

jQuery.fn.extend({
    getMaxZ : function(){
        return Math.max.apply(null, jQuery(this).map(function(){
            var z;
            return isNaN(z = parseInt(jQuery(this).css("z-index"), 10)) ? 0 : z;
        }));
    }
});

Usage:

$("#layer-1,#layer-2,#layer-3,#layer-4").getMaxZ();

Answer 2

Here how I got both lowest/highest z-indexes. If you only want to get the highest z-index and nothing more, then this function may not efficient, but if you want to get all z-indexes and the ids associated with it (i.e. for use with bring 'layer' to front/send to back, bring forward, send backward, etc), this is one way to do it. The function returns an array of objects containing ids and their z-indexes.

function getZindex (id) {

     var _l = [];
     $(id).each(function (e) {
         // skip if z-index isn't set 
         if ( $(this).css('z-index') == 'auto' ) {
              return true
         }
         _l.push({ id: $(this), zindex: $(this).css('z-index') });
     });
     _l.sort(function(a, b) { return a.zindex - b.zindex });
     return _l;
}

// You'll need to add a class 'layer' to each of your layer
var _zindexes = getZindex('.layer');
var _length = _zindexes.length;

// Highest z-index is simply the last element in the array
var _highest = _zindexes[_length - 1].zindex

// Lowest z-index is simply the first element in the array
var _lowest = _zindex[0].zindex;

alert(_highest);
alert(_lowest);

Answer 3

Vanilla JS, not 100% cross-browser. Including as reference for future readers/alternative method.

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;
}

Answer 4

Besides @justkt's native solution above, there is a nice plugin to do what you want. Take a look at TopZIndex.

$.topZIndex("div");

Answer 5

I don't know how efficient this is, but you can use $.map to get all the z-indices:

var $divs = $('div'),
    mapper = function (elem) {
        return parseFloat($(elem).css('zIndex'));
    },
    indices = $.map($divs, mapper);

The indices variable is now an array of all the z-indices for all the divs. All you'd have to do now is apply them to Math.max:

var highest = Math.max.apply(whatevs, indices);