function getOffsetSum(ele){
  var top= 0,left=0;
  while(ele){
    top+=ele.offsetTop;
    left+=ele.offsetLeft;
    ele=ele.offsetParent;
  }
  return {
    top:top,
    left:left
  }
}

function getOffsetRect(ele){
      var box=ele.getBoundingClientRect();
      var body=document.body,
        docElem=document.documentElement;
      //获取页面的scrollTop,scrollLeft(兼容性写法)
      var scrollTop=window.pageYOffset||docElem.scrollTop||body.scrollTop,
        scrollLeft=window.pageXOffset||docElem.scrollLeft||body.scrollLeft;
      var clientTop=docElem.clientTop||body.clientTop,
        clientLeft=docElem.clientLeft||body.clientLeft;
      var top=box.top+scrollTop-clientTop,
        left=box.left+scrollLeft-clientLeft;
      return {
        //Math.round 兼容火狐浏览器bug
        top:Math.round(top),
        left:Math.round(left)
      }
    }

//获取元素相对于页面的偏移
function getOffset(ele){
  if(ele.getBoundingClientRect){
    return getOffsetRect(ele);
  }else{
    return getOffsetSum(ele);
  }
}