How do I check if an element is really visible with JavaScript?

For the point 2.

I see that no one has suggested to use document.elementFromPoint(x,y), to me it is the fastest way to test if an element is nested or hidden by another. You can pass the offsets of the targetted element to the function.

Here's PPK test page on elementFromPoint.

From MDN's documentation:

The elementFromPoint() method—available on both the Document and ShadowRoot objects—returns the topmost Element at the specified coordinates (relative to the viewport).

Interesting question.

This would be my approach.

1. At first check that element.style.visibility !== 'hidden' && element.style.display !== 'none'
2. Then test with document.elementFromPoint(element.offsetLeft, element.offsetTop) if the returned element is the element I expect, this is tricky to detect if an element is overlapping another completely.
3. Finally test if offsetTop and offsetLeft are located in the viewport taking scroll offsets into account.

Hope it helps.

This is what I have so far. It covers both 1 and 3. I'm however still struggling with 2 since I'm not that familiar with Prototype (I'm more a jQuery type of guy).

function isVisible( elem ) {
    var $elem = $(elem);

    // First check if elem is hidden through css as this is not very costly:
    if ($elem.getStyle('display') == 'none' || $elem.getStyle('visibility') == 'hidden' ) {
        //elem is set through CSS stylesheet or inline to invisible
        return false;
    }

    //Now check for the elem being outside of the viewport
    var $elemOffset = $elem.viewportOffset();
    if ($elemOffset.left < 0 || $elemOffset.top < 0) {
        //elem is left of or above viewport
        return false;
    }
    var vp = document.viewport.getDimensions();
    if ($elemOffset.left > vp.width || $elemOffset.top > vp.height) {
        //elem is below or right of vp
        return false;
    }

    //Now check for elements positioned on top:
    //TODO: Build check for this using Prototype...
    //Neither of these was true, so the elem was visible:
    return true;
}

Prototype's Element library is one of the most powerful query libraries in terms of the methods. I recommend you to check out the API.

A few hints:

1. Checking visibility can be a pain, but you can use the Element.getStyle() method and Element.visible() methods combined into a custom function. With getStyle() you can check the actual computed style.

2. I don't know exactly what you mean by "underneath" :) If you meant by it has a specific ancestor, for example, a wrapper div, you can use Element.up(cssRule):

   var child = $("myparagraph");
   if(!child.up("mywrapper")){
     // I lost my mom!
   }
   else {
     // I found my mom!
   }
   

   If you want to check the siblings of the child element you can do that too:

   var child = $("myparagraph");
   if(!child.previous("mywrapper")){
     // I lost my bro!
   } 
   else {
     // I found my bro!
   }
   

3. Again, Element lib can help you if I understand correctly what you mean :) You can check the actual dimensions of the viewport and the offset of your element so you can calculate if your element is "off screen".

Good luck!

I pasted a test case for prototypejs at http://gist.github.com/117125. It seems in your case we simply cannot trust in getStyle() at all. For maximizing the reliability of the isMyElementReallyVisible function you should combine the following:

• Checking the computed style (dojo has a nice implementation that you can borrow)
• Checking the viewportoffset (prototype native method)
• Checking the z-index for the "beneath" problem (under Internet Explorer it may be buggy)

Catch mouse-drag and viewport events (onmouseup, onresize, onscroll).

When a drag ends do a comparison of the dragged item boundary with all "elements of interest" (ie, elements with class "dont_hide" or an array of ids). Do the same with window.onscroll and window.onresize. Mark any elements hidden with a special attribute or classname or simply perform whatever action you want then and there.

The hidden tests are pretty easy. For "totally hidden" you want to know if ALL corners are either inside the dragged-item boundary or outside the viewport. For partially hidden you're looking for a single corner matching the same test.

Check elements' offsetHeight property. If it is more than 0, it is visible. Note: this approach doesn't cover a situation when visibility:hidden style is set. But that style is something weird anyways.

Here is a sample script and test case. Covers positioned elements, visibilty: hidden, display: none. Didn't test z-index, assume it works.

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
  <head>
    <title></title>
    <style type="text/css">
    div {
      width: 200px;
      border: 1px solid red;
    }
    p {
      border: 2px solid green;
    }
    .r {
      border: 1px solid #BB3333;
      background: #EE9999;
      position: relative;
      top: -50px;
      height: 2em;
    }
    .of {
      overflow: hidden;
      height: 2em;
      word-wrap: none; 
    }
    .of p {
      width: 100%;
    }

    .of pre {
      display: inline;
    }
    .iv {
      visibility: hidden;
    }
    .dn {
      display: none;
    }
    </style>
    <script src="http://www.prototypejs.org/assets/2008/9/29/prototype-1.6.0.3.js"></script>
    <script>
      function isVisible(elem){
        if (Element.getStyle(elem, 'visibility') == 'hidden' || Element.getStyle(elem, 'display') == 'none') {
          return false;
        }
        var topx, topy, botx, boty;
        var offset = Element.positionedOffset(elem);
        topx = offset.left;
        topy = offset.top;
        botx = Element.getWidth(elem) + topx;
        boty = Element.getHeight(elem) + topy;
        var v = false;
        for (var x = topx; x <= botx; x++) {
          for(var y = topy; y <= boty; y++) {
            if (document.elementFromPoint(x,y) == elem) {
              // item is visible
              v = true;
              break;
            }
          }
          if (v == true) {
            break;
          }
        }
        return v;
      }

      window.onload=function() {
        var es = Element.descendants('body');
        for (var i = 0; i < es.length; i++ ) {
          if (!isVisible(es[i])) {
            alert(es[i].tagName);
          }
        }
      }
    </script>
  </head>
  <body id='body'>
    <div class="s"><p>This is text</p><p>More text</p></div>
    <div class="r">This is relative</div>
    <div class="of"><p>This is too wide...</p><pre>hidden</pre>
    <div class="iv">This is invisible</div>
    <div class="dn">This is display none</div>
  </body>
</html>