抽取jquery源码实现获取元素的精确位置

获取元素准确位置的写法
知识点预备：getBoundingClientRect()
clientTop,left：边框的距离
scrollTop，scrollLeft:到上边或者左边的滚动距离
pageYOffset：firefox 滚动条顶部到网页上边的距离
pageXOffset：firefox 滚动条顶部到网页左边的距离

var getCoords = function(el){ 
var box = el.getBoundingClientRect(), 
doc = el.ownerDocument, 
body = doc.body, 
html = doc.documentElement, 
clientTop = html.clientTop || body.clientTop || 0, 
clientLeft = html.clientLeft || body.clientLeft || 0, 
top = box.top + (self.pageYOffset || html.scrollTop || body.scrollTop ) - clientTop, 
left = box.left + (self.pageXOffset || html.scrollLeft || body.scrollLeft) - clientLeft 
return { 'top': top, 'left': left }; 
};