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html - Javascript: document.getElementById() returns NULL

I am quite new with Javascript and I got a problem with document.getElementById() that always returns NULL, and that's driving me nuts.

I have a element in my code and I want to get its coordinates so I can move it.

Here is the code:

<html>
  <head>
    <script type="text/javascript" >
      function MoveIt(obj) {
        alert(obj); // returns "Object HTMLDivElement"
        var xx = document.getElementById("arect");

        if(document.getElementById("arect").value == null) {
          alert('NULL >> ' + xx.value);
        }
        else {
          alert('NOT NULL >>' + xx.value);
        }

        posX = xx.style.scrollTop;
        posY = xx.style.left;
      }
    </script>
  </head>

  <body bgcolor="white" >
    <DIV class="background" id="MyDiv2">  
      <div id="arect" name="arect" class="transbox" onmousedown="MoveIt(this);" >
      </div>
    </div>
  </body>
</html>

The above function MoveIt() always returns NULL

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1 Answer

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by (71.8m points)

The page contents need to be loaded before trying to read them. Try

window.onload = function() {
  // run your script in here
}

Or if you're using jQuery, prefer

$(document).ready(function() {
  ...
}

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