得到元素相对于document的offset

getOffset

/**
 * 得到元素相对于document的offset
 * @param  {Element} ele
 * @return {object}
 */
function getOffset(ele) {
    var box = {top: 0, left: 0};
    var doc = ele.ownerDocument;
    var docElem = doc.documentElement;
    var win = doc.defaultView;

    if (ele.getBoundingClientRect) {
        box = ele.getBoundingClientRect();
    }

    return {
        top: box.top + win.pageYOffset - docElem.clientTop,
        left: box.left + win.pageXOffset - docElem.clientLeft
    };
}