Click And Drag function in javascript

Question

I'm trying to create my own click and drag function in JavaScript without the use of jquery. I know that jquery is easy to implement, but I prefer my own code. What I have, as i click the div, then move the mouse, the div moves to the same spot and doesn't implement a "dragging" look to it. I'm not sure why this is. I want my outcome to be able to move the div over the image that way I can "crop" the image based on the div, etc. My code is:

index.js

function _(element) {
  return document.getElementById(element);
}

index.css

body {
  background-color: rgb(33, 66, 99);
  margin: 0px;
  padding: 0px;
}

img {
  position:absolute;
}

.selection {
  width: 200px;
  height: 200px;
  background-color: rgb(255,255,255);
  position: absolute;
}

index.php

<!DOCTYPE html>
<html>
  <head>
    <meta charset = "UTF-8"/>
    <title>Image Cropping</title>
    <link rel = "stylesheet" href = "index.css"/>
    <script src = "index.js"></script>
  </head>
  <body>
    <div class = "image">
      <img src = "model.jpg" alt = "Model" id = "theImage"/>
      <div class = "selection" id = "selection"/>
    </div>
    <script>
      _("theImage").ondragstart = function() { return false; };
      var m = _("selection");
      m.addEventListener("mousedown", mouseDown, false);
      window.addEventListener("mouseup", mouseUp, false);

      function mouseUp() {
        window.removeEventListener("mousemove", move, true);
      }

      function mouseDown(e) {
        window.addEventListener("mousemove", move, true);
      }

      function move(e) {
        var x = m.style.left;
        var y = m.style.top;

        var mouseX = e.clientX;
        var mouseY = e.clientY;

        m.style.top += (mouseX - x) + "px";
        m.style.left += (mouseY - y) + "px";

        // Also tried: m.style.top = (mouseX - x) + "px";
        // And       : m.style.left = (mouseY - y) + "px";
      }
    </script>
  </body>
</html>

Try the Chuck Norris way -- how to implement a drag-and-drop div from scratch? — WhiteHat
– WhiteHat, Commented Aug 17, 2015 at 19:55

blex · Accepted Answer · 2015-08-17 20:20:53Z

To add the "dragging look to it", you can:

change the cursor (cursor: move;)
keep the cursor's offset relative to the mouse

For the second one, I reused a function I created for one of my projects, for which I implemented drag and drop for mobile, not wanting to use a big library:

/*
 * Returns the given element's offset relative to the document.
 */
function realOffset(elem) {
    var top = 0, left = 0;
    while (elem) {
        top = top + parseInt(elem.offsetTop, 10);
        left = left + parseInt(elem.offsetLeft, 10);
        elem = elem.offsetParent;
    }
    return { top: top, left: left };
}

Using this function, the math becomes simple:

m.style.left = (mouseX - offset.left) + "px";
m.style.top  = (mouseY - offset.top) + "px";

Full demo

_("theImage").ondragstart = function () { return false; };

var m = _("selection"), offset;
m.addEventListener("mousedown", mouseDown, false);
window.addEventListener("mouseup", mouseUp, false);

function mouseUp() { window.removeEventListener("mousemove", move, true); }

function mouseDown(e) {
    // SAVE THE OFFSET HERE
    offset = {
    	left: e.pageX - realOffset(m).left,
        top: e.pageY - realOffset(m).top
    };
    window.addEventListener("mousemove", move, true);
}

function move(e) {
    // REUSE THE OFFSET HERE
    m.style.left  = (e.pageX - offset.left) + "px";
    m.style.top = (e.pageY - offset.top) + "px";
}

/*
 * Returns the given element's offset relative to the document.
 */
function realOffset(elem) {
    var top = 0, left = 0;
    while (elem) {
        top = top + parseInt(elem.offsetTop, 10);
        left = left + parseInt(elem.offsetLeft, 10);
        elem = elem.offsetParent;
    }
    return { top: top, left: left };
}

function _(element) { return document.getElementById(element); }

body {
  background-color: rgb(33, 66, 99);
  margin: 0px;
  padding: 0px;
}

img {
  position:absolute;
}

.selection {
  width: 200px;
  height: 200px;
  background-color: rgba(255,255,255,.5);
  position: absolute;
  cursor: move;
}

<div class="image">
    <img src="http://i.imgur.com/vxkljMP.jpg" alt="Model" id="theImage" />
    <div class="selection" id="selection"></div>
</div>

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