code: function getPositionFromOffset(el, of) {
    var offsetParent = getOffsetParent(el);
    var parentOf = getOffset(offsetParent);
    return {
      x: of.x - parentOf.x,
      y: of.y - parentOf.y
    };
  }
docs: get position of a el if its offset is given. like jQuery.offset. 类似 jQuery.offset的设置功能, 但是它只返回计算的position, 不改变元素样式.