js 获取元素相对于窗口的定位

var box = document.getElementById("box");

var pos = box.getBoundingClientRect();//此方法
box.innerHTML = "top:"+pos.top +
  "left:"+pos.left +
  "bottom:"+pos.bottom +
  "right:"+pos.right +
  "width:"+pos.width +
  "height:"+pos.height