How to implement a drag-and-drop div from scratch with JavaScript?

Question

It should be a combination of CSS and JavaScript. The steps to do should be:

1. Make it on top of all other elements (which property to specify?)
2. Catch t

Answer 1

1. make it absolute positioned, with a high z-index.
2. check for onmousedown of the div.
3. use the event's mouseX and mouseY attributes to move the div.

Here's an example from Javascript, the Definitive Guide (updated here):

/**
 *  Drag.js:    drag absolutely positioned HTML elements.
 *
 *  This module defines a single drag() function that is designed to be called
 *  from an onmousedown event handler. Subsequent mousemove event will
 *  move the specified element. A mouseup event will terminate the drag.
 *  If the element is dragged off the screen, the window does not scroll.
 *  This implementation works with both the DOM Level 2 event model and the
 *  IE event model.
 *
 *  Arguments:
 *
 *      elementToDrag: the element that received the mousedown event or
 *          some containing element. It must be absolutely positioned. Its
 *          style.left and style.top values will be changed based on the user's
 *          drag.
 *
 *      event: ethe Event object for the mousedown event.
 *
 *  Example of how this can be used:
 *      <script src="Drag.js"></script> <!-- Include the Drag.js script -->
 *      <!-- Define the element to be dragged -->
 *      <div style="postion:absolute; left:100px; top:100px; width:250px;
 *                  background-color: white; border: solid black;">
 *      <!-- Define the "handler" to drag it with. Note the onmousedown attribute. -->
 *      <div style="background-color: gray; border-bottom: dotted black;
 *                  padding: 3px; font-family: sans-serif; font-weight: bold;"
 *          onmousedown="drag(this.parentNode, event);">
 *      Drag Me <!-- The content of the "titlebar" -->
 *      </div>
 *      <!-- Content of the draggable element -->
 *      <p>This is a test. Testing, testing, testing.<p>This is a test.<p>Test.
 *      </div>
 *
 *  Author: David Flanagan; Javascript: The Definitive Guide (O'Reilly)
 *  Page: 422
 **/
 function drag(elementToDrag, event)
 {
     // The mouse position (in window coordinates)
     // at which the drag begins
     var startX = event.clientX, startY = event.clientY;

     // The original position (in document coordinates) of the
     // element that is going to be dragged. Since elementToDrag is
     // absolutely positioned, we assume that its offsetParent is the
     //document bodt.
     var origX = elementToDrag.offsetLeft , origY = elementToDrag.offsetTop;

     // Even though the coordinates are computed in different
     // coordinate systems, we can still compute the difference between them
     // and use it in the moveHandler() function. This works because
     // the scrollbar positoin never changes during the drag.
     var deltaX = startX - origX, deltaY = startY - origY;

     // Register the event handlers that will respond to the mousemove events
     // and the mouseup event that follow this mousedown event.
     if (document.addEventListener) //DOM Level 2 event model
     {
         // Register capturing event handlers
         document.addEventListener("mousemove", moveHandler, true);
         document.addEventListener("mouseup", upHandler, true);
     }
     else if (document.attachEvent) //IE 5+ Event Model
     {
         //In the IE event model, we capture events by calling
         //setCapture() on the element to capture them.
         elementToDrag.setCapture();
         elementToDrag.attachEvent("onmousemove", moveHandler);
         elementToDrag.attachEvent("onmouseup", upHandler);
         // Treat loss of mouse capture as a mouseup event.
         elementToDrag.attachEvent("onclosecapture", upHandler);
     }
     else //IE 4 Event Model
     {
         // In IE 4, we can't use attachEvent() or setCapture(), so we set
         // event handlers directly on the document object and hope that the
         // mouse event we need will bubble up.
         var oldmovehandler = document.onmousemove; //used by upHandler()
         var olduphandler = document.onmouseup;
         document.onmousemove = moveHandler;
         document.onmouseup = upHandler;
     }

     // We've handled this event. Don't let anybody else see it.
     if (event.stopPropagation) event.stopPropagation();    //  DOM Level 2
     else event.cancelBubble = true;                        //  IE

     // Now prevent any default action.
     if (event.preventDefault) event.preventDefault();      //  DOM Level 2
     else event.returnValue = false;                        //  IE

     /**
      * This is the handler that captures mousemove events when an element
      * is being dragged. It is responsible for moving the element.
      **/
      function moveHandler(e)
      {
          if (!e) e = window.event; //  IE Event Model

          // Move the element to the current mouse position, adjusted as
          // necessary by the offset of the initial mouse-click.
          elementToDrag.style.left = (e.clientX - deltaX) + "px";
          elementToDrag.style.top = (e.clientY - deltaY) + "px";

          // And don't let anyone else see this event.
          if (e.stopPropagation) e.stopPropagation();       // DOM Level 2
          else e.cancelBubble = true;                       // IE
      }

      /**
       * This is the handler that captures the final mouseup event that
       * occurs at the end of a drag.
       **/
       function upHandler(e)
       {
           if (!e) e = window.event;    //IE Event Model

           // Unregister the capturing event handlers.
           if (document.removeEventListener) // DOM event model
            {
                document.removeEventListener("mouseup", upHandler, true);
                document.removeEventListener("mousemove", moveHandler, true);
            }
            else if (document.detachEvent)  //  IE 5+ Event Model
            {
                elementToDrag.detachEvent("onlosecapture", upHandler);
                elementToDrag.detachEvent("onmouseup", upHandler);
                elementToDrag.detachEvent("onmousemove", moveHandler);
                elementToDrag.releaseCapture();
            }
            else    //IE 4 Event Model
            {
                //Restore the original handlers, if any
                document.onmouseup = olduphandler;
                document.onmousemove = oldmovehandler;
            }

            //  And don't let the event propagate any further.
            if (e.stopPropagation) e.stopPropagation(); //DOM Level 2
            else e.cancelBubble = true;                 //IE
       }
 }

 function closeMe(elementToClose)
 {
     elementToClose.innerHTML = '';
     elementToClose.style.display = 'none';
 }

 function minimizeMe(elementToMin, maxElement)
 {
     elementToMin.style.display = 'none';
 }

Answer 2

HTML5 Drag and Drop

If you are reading this in the year 2017 or later, you might want to have a look at the HTML5 Drag and Drop API:

https://developer.mozilla.org/docs/Web/API/HTML_Drag_and_Drop_API

Example:

<!DOCTYPE HTML>
<html>
<head>
    <script>
    function allowDrop(ev) {
        ev.preventDefault();
    }

    function drag(ev) {
        ev.dataTransfer.setData("text", ev.target.id);
    }

    function drop(ev) {
        ev.preventDefault();
        var data = ev.dataTransfer.getData("text");
        ev.target.appendChild(document.getElementById(data));
    }
    </script>
    <style>
        .draggable {
            border: 1px solid black;
            width: 30px;
            height: 20px;
            float: left;
            margin-right: 5px;
        }
        #target {
            border: 1px solid black;
            width: 150px;
            height: 100px;
            padding: 5px;
        }
    </style>
</head>
<body>

    <h1>Drag and Drop</h1>

    <h2>Target</h2>
    <div id="target" ondrop="drop(event)" ondragover="allowDrop(event)"></div>

    <h2>Draggable Elements</h2>
    <div id="draggable1" class="draggable" draggable="true" ondragstart="drag(event)"></div>
    <div id="draggable2" class="draggable" draggable="true" ondragstart="drag(event)"></div>
    <div id="draggable3" class="draggable" draggable="true" ondragstart="drag(event)"></div>

</body>
</html>