获取页面元素的位置

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 };
};

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