How can you figure out the highest z-index in your document?

You could call findHighestZIndex for a particular element type such as <div> like this:

findHighestZIndex('div');

assuming a findHighestZindex function that is defined like this:

function findHighestZIndex(elem)
{
  var elems = document.getElementsByTagName(elem);
  var highest = Number.MIN_SAFE_INTEGER || -(Math.pow(2, 53) - 1);
  for (var i = 0; i < elems.length; i++)
  {
    var zindex = Number.parseInt(
      document.defaultView.getComputedStyle(elems[i], null).getPropertyValue("z-index"),
      10
    );
    if (zindex > highest)
    {
      highest = zindex;
    }
  }
  return highest;
}

Stealing some code from abcoder site for the sake of clarity:

  var maxZ = Math.max.apply(null, 
    $.map($('body *'), function(e,n) {
      if ($(e).css('position') != 'static')
        return parseInt($(e).css('z-index')) || 1;
  }));

I’d like to add my ECMAScript 6 implementation that I use in one of my UserScripts. I’m using this one to define the z-index of specific elements so that they always appear the highest.

In JS, you can additionally set certain attributes or class names to elements that you may want to exclude. For instance, consider your script setting a data-highest attribute on an element that you want to appear as the highest element (e.g. a popup); and consider an element with class name yetHigher that you don’t control, which should be even higher (e.g. a custom context menu). I can exclude these elements with a chained :not selector. Note that :not([data-highest], .yetHigher) is possible, but experimental, and only has limited browser support as of January 2021.

let highestZIndex = 0;

// Then later, potentially repeatedly
highestZIndex = Math.max(
  highestZIndex,
  ...Array.from(document.querySelectorAll("body *:not([data-highest]):not(.yetHigher)"), (elem) => parseFloat(getComputedStyle(elem).zIndex))
    .filter((zIndex) => !isNaN(zIndex))
);

The lower five lines can run multiple times and update the variable highestZIndex repeatedly by finding out the maximum between the current highestZIndex value and all the other computed z-indexes of all elements. The filter excludes all the "auto" values.

Using ES6 a cleaner approach

function maxZIndex() {

     return Array.from(document.querySelectorAll('body *'))
           .map(a => parseFloat(window.getComputedStyle(a).zIndex))
           .filter(a => !isNaN(a))
           .sort()
           .pop();
}