js擷取元素的座標點

/**
 * 擷取元素的座標點
 * @param elementId 元素Id
 * @returns 元素所在位置的座標點
 */
function getElementPos(elementId)
{  
 var ua = navigator.userAgent.toLowerCase();  
 var isOpera = (ua.indexOf('opera') != -1);  
 var isIE = (ua.indexOf('msie') != -1 && !isOpera); // not opera spoof  
 var el = document.getElementById(elementId);  
 if(el.parentNode === null || el.style.display == 'none')
 {  
  return false;  
 }        
 var parent = null;  
 var pos = [];       
 var box;       
 if(el.getBoundingClientRect)    //IE  
 {           
   box = el.getBoundingClientRect();  
   var scrollTop = Math.max(document.documentElement.scrollTop, document.body.scrollTop);  
   var scrollLeft = Math.max(document.documentElement.scrollLeft, document.body.scrollLeft);  
   return {x:box.left + scrollLeft, y:box.top + scrollTop};  
 }
 else if(document.getBoxObjectFor)    // gecko      
 {  
   box = document.getBoxObjectFor(el);   
   var borderLeft = (el.style.borderLeftWidth)?parseInt(el.style.borderLeftWidth):0;   
   var borderTop = (el.style.borderTopWidth)?parseInt(el.style.borderTopWidth):0;   
   pos = [box.x - borderLeft, box.y - borderTop];  
 }
 else    // safari & opera
 {  
   pos = [el.offsetLeft, el.offsetTop];    
   parent = el.offsetParent;       
   if (parent != el)
   {   
     while (parent)
     {    
      pos[0] += parent.offsetLeft;   
      pos[1] += parent.offsetTop;   
      parent = parent.offsetParent;  
    }    
  }     
  if (ua.indexOf('opera') != -1 || ( ua.indexOf('safari') != -1 && el.style.position == 'absolute' ))
  {   
    pos[0] -= document.body.offsetLeft;  
    pos[1] -= document.body.offsetTop;           
   }      
 }                
  if (el.parentNode)
  {   
     parent = el.parentNode;  
   }
  else
  {  
     parent = null;  
   }  
  while (parent && parent.tagName != 'BODY' && parent.tagName != 'HTML') { // account for any scrolled ancestors  
  pos[0] -= parent.scrollLeft;  
  pos[1] -= parent.scrollTop;  
  if (parent.parentNode)
  {  
    parent = parent.parentNode;  
  }
  else
  {  
    parent = null;  
   }  
  }  
  return {x:pos[0], y:pos[1]};  
}