js精确定位HTML标签的TOP和LEFT值

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///回一个元素相对于整个文档左上角的坐标，即元素的 top 、left 的位置
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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];