function offsetTop( elements ){

var top = elements.offsetTop;

var parent = elements.offsetParent;

while( parent != null ){

top += parent.offsetTop;

parent = parent.offsetParent;

};

return top;

};

 function getTop(e){

var offset=e.offsetTop;

if(e.offsetParent!=null)

offset+=getTop(e.offsetParent);

return offset;

}

element.getBoundingClientRect()   获取元素距页面顶端距离。