获取页面上每个元素的Z索引

Question

I am trying to find the highest z-index on a page. I am using this

var getStyleProp = function(el, prop){
            return window.getComputedStyle(el, null).getPropertyValue(prop);
}
var getHighestZIndex = function(){

            var highestZIndex = 0,
                HTMLElems = ["a","abbr","acronym","address","applet","area","article","audio","b","base","basefont","bdi","bdo","big","blink","blockquote","body","br","button","canvas","caption","center","cite","code","col","colgroup","content","data","datalist","dd","decorator","del","details","dfn","dialog","dir","div","dl","dt","element","em","embed","fieldset","figcaption","figure","footer","form","frame","frameset","h1, h2, h3, h4, h5, h6","head","header","hgroup","hr","html","i","iframe","img","input","ins","isindex","kbd","keygen","label","legend","li","link","listing","main","map","mark","menu","menuitem","meta","meter","nav","noembed","noscript","object","ol","optgroup","option","output","p","param","plaintext","pre","progress","q","rp","rt","rtc","ruby","s","samp","script","section","select","shadow","small","source","spacer","span","strike","strong","style","sub","summary","sup","table","tbody","td","template","textarea","tfoot","th","thead","time","title","tr","track","tt","u","ul","var","video","wbr","xmp"],
                tags,
                zIndex;

            for (var i = 0; i < HTMLElems.length; i++){

                tags = document.getElementsByTagName(HTMLElems[i]);

                if (tags){
                    for (var c = 0; c < tags.length; c++){
                        zIndex =getStyleProp(tags[c], "z-index");
                           console.log(tags[c], "zIndex=", zIndex);
                        if (zIndex > highestZIndex){
                            highestZIndex = zIndex;
                        }

                    }   
                }


            }

            return highestZIndex;

        }
console.log(getHighestZIndex());

But everything is coming back as "auto" . This ancient article explains how a "bug" is causing this behavior. I've tried to make clones of each node, set the position to relative, and then get the z-index,

cl.style.display = "none";
cl.style.position = "absolute";
zIndex = (getStyleProp(cl, "z-index"));

but that is not working either. What is wrong here? Is there a cheaper way to do this than recreating everything on the page?

JSBIN

Answer 1

The node's clone does not seem to get the z-index, while the node itself returns the right value. You could try using it instead (not sure how it might react on a page with lots of content):

var getHighestZIndex = function () {
    var highestZIndex = 0,
        zIndex,
        pos,
        tags = document.body.getElementsByTagName('*');

    for (var c = 0; c < tags.length; c++) {
        // Get the original 'position' property
        pos = getComputedStyle(tags[c]).position;
        // Set it temporarily to 'relative'
        tags[c].style.position = "relative";
        // Grab the z-index
        zIndex = getComputedStyle(tags[c]).zIndex;
        // Reset the 'position'
        tags[c].style.position = pos;

        console.log(tags[c], "zIndex=", zIndex);

        if (zIndex > highestZIndex) { highestZIndex = zIndex; }
    }

    return highestZIndex;
};

console.log(getHighestZIndex());

JS Fiddle Demo

Changing the element's position temporarily might produce a glitch. You'll need to test that on a page with lots of contents and elements that are position:absolute; or position:fixed; .

Answer 2

If this doesn't fit your use-case, just let me know, and I'll remove it. However, as a thought.

Can you loop through all the tags, and if the value is "auto" assume it's 999. If it's >= 1000, take that as your "highest" value. Then, increment your zIndex up from your highest number that way. This way, the first tag you place will be 1000, the next will be 1001.

var elements  = document.getElementsByTagName("*"),
    maxZIndex = 999;

for( var i=0; i < elements.length; i++ ) {
    var zIndex = parseInt(elements[0].style.zIndex) || 999;
    maxZIndex  = zIndex > maxZIndex ? zIndex : maxZIndex;
}
return maxZIndex;

This would fit a use case where you are just trying to make sure that the tag you are placing is greater than anything on the page...such as placing a modal.

999 is overkill...somewhat of a "just in case" I missed something because anything with z-index:auto is equivalent to zero. See the following "proof" where even though my z-index is only "1" it overlaps boxes that are 3-deep of "auto".

<div style='position:absolute;background-color:white;z-index:1;width:94px;height:94px;'>

</div>
<div style='position:absolute;background-color:red;width:100px;height:100px;'>
    <div style='position:absolute;background-color:blue;width:98px;height:98px;'>
        <div style='position:absolute;background-color:green;width:96px;height:96px;'>

        </div>
    </div>
</div>