Javascript/event.client

var flg = false;
var x = 0;
var y = 0;
var obj = document.getElementById("obj");
obj.onmousedown = function(e) {
  flg = true;
  x = e.clientX - parseInt(obj.style.left);
  y = e.clientY - parseInt(obj.style.top);
}
// end drag
obj.onmouseup = function(e) {
  flg = false;
}
// drag 
window.onmousemove = function(e) {
  if(flg == true) {
    obj.style.left = (e.clientX - x) + "px";
    obj.style.top = (e.clientY - y) + "px";    
  }
}