SOURCE

var drag = document.getElementById('drag');

var flag = false,diffX,diffY;
drag.onmousedown = function(e) {
 var e = e || window.event;
  flag = true;
 diffX = e.clientX - drag.offsetLeft;
diffY = e.clientY - drag.offsetTop;
}

document.onmousemove = function(e) {
  
  if(!flag) return;
  var e = e || window.event;
  var left = e.clientX-diffX;
  var top = e.clientY-diffY;
  if(left < 0 ) {
    left = 0
  }else if(left > window.innerWidth - drag.offsetWidth) {
    left = window.innerWidth - drag.offsetWidth;
  }
  if(top < 0 ) {
    top = 0
  }else if(top > window.innerHeight - drag.offsetHeight) {
    top = window.innerHeight - drag.offsetHeight;
  }
  
  drag.style.left = left+'px';
  drag.style.top = top + 'px';
}

document.onmouseup = function(e) {
  flag = false;
}

<div class="calculator" id="drag">**********11</div>  

.calculator{
  background:green;
  height:200px;
  width:300px;
  position:absolute;
  cursor:move;
}