movable gui javascript

JavaScript
<script>
  var dragged;

  /* events fired on the draggable target */
  document.addEventListener("drag", function(event) {

  }, false);

  document.addEventListener("dragstart", function(event) {
    // store a ref. on the dragged elem
    dragged = event.target;
    // make it half transparent
    event.target.style.opacity = .5;
  }, false);

  document.addEventListener("dragend", function(event) {
    // reset the transparency
    event.target.style.opacity = "";
  }, false);

  /* events fired on the drop targets */
  document.addEventListener("dragover", function(event) {
    // prevent default to allow drop
    event.preventDefault();
  }, false);

  document.addEventListener("dragenter", function(event) {
    // highlight potential drop target when the draggable element enters it
    if (event.target.className == "dropzone") {
      event.target.style.background = "purple";
    }

  }, false);

  document.addEventListener("dragleave", function(event) {
    // reset background of potential drop target when the draggable element leaves it
    if (event.target.className == "dropzone") {
      event.target.style.background = "";
    }

  }, false);

  document.addEventListener("drop", function(event) {
    // prevent default action (open as link for some elements)
    event.preventDefault();
    // move dragged elem to the selected drop target
    if (event.target.className == "dropzone") {
      event.target.style.background = "";
      dragged.parentNode.removeChild( dragged );
      event.target.appendChild( dragged );
    }
  }, false);

</script><!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>jQuery UI Draggable - Default functionality</title>
  <link rel="stylesheet" href="//code.jquery.com/ui/1.12.1/themes/base/jquery-ui.css">
  <link rel="stylesheet" href="/resources/demos/style.css">
  <style>
  #draggable { width: 150px; height: 150px; padding: 0.5em; }
  </style>
  <script src="https://code.jquery.com/jquery-1.12.4.js"></script>
  <script src="https://code.jquery.com/ui/1.12.1/jquery-ui.js"></script>
  <script>
  $( function() {
    $( "#draggable" ).draggable();
  } );
  </script>
</head>
<body>
 
<div id="draggable" class="ui-widget-content">
  <p>Drag me around</p>
</div>
 
 
</body>
</html>
Source