Edit in JSFiddle

    (function(){
      function getStyle(elem){
        return getComputedStyle(elem);
      }
      function parsePx(str){
        if(/px/.test(str)){
          str = str.replace(/px/,'');
        }
        return parseFloat(str);
      }
      var drag = document.querySelector('.drag');
      var top,
        left,
        currentX,
        currentY,
        moving = false;
      drag.onmousedown = function(event){
        var css = getStyle(drag);
        top = parsePx(css.top);
        left = parsePx(css.left);
        //console.log('down',top,left);
        currentX = event.clientX;
        currentY = event.clientY;
        moving = true
      };
      document.onmouseup = function(event){
        //console.log('up');
        moving = false;
        currentX = currentY = top = left =0;
      };
      document.onmousemove = function(event){
        if(!moving)return;
        drag.style.top = top + (event.clientY - currentY) + 'px';
        drag.style.left = left + (event.clientX - currentX) + 'px';
      };
			/*

      drag.onmouseleave = function(event){
				 if(!moving)return;
        drag.style.top = top + (event.clientY - currentY) + 'px';
        drag.style.left = left + (event.clientX - currentX) + 'px';
        moving = false;
        currentX = currentY = top = left =0;
      };
*/
    })()

<div id="app">
    <div class="drag"></div>
  </div>

#app{
      width: 800px;
      margin:50px auto;
      position: relative;
    }
    .drag{
      position: absolute;
      top: 20px;
      left: 30px;
      cursor: pointer;
      border: 1px solid #000;
      padding: 5px;
      width:20px;
      height:20px;
      background-color:red;
    }