getBoundingClientRect()方法。它返回一个对象，其中包含了left、right、top、bottom四个属性，分别对应了该元素的左上角和右下角相对于浏览器窗口（viewport）左上角的距离。
通用的获得元素相对于viewPort的函数,一直向上遍历父元素（因为有些浏览器不支持getBoundingClientRect方法的）：
function getElementLeft(element) {
var actualLeft = element.offsetLeft;
var current = element.offsetParent;
while (current !== null) {
actualLeft += current.offsetLeft;
current = current.offsetParent;
}
return actualLeft;
}
function getElementTop(element) {
var actualTop = element.offsetTop;
var current = element.offsetParent;
while (current !== null) {
actualTop += current.offsetTop;
current = current.offsetParent;
}
return actualTop;
}
function getBoundingClientRect(element) {
var scrollTop = document.documentElement.scrollTop;
var scrollLeft = document.documentElement.scrollLeft;
if (element.getBoundingClientRect) {
if (typeof arguments.callee.offset != “number”){ //修复ie8以及之前版本从（2,2）坐标开始计算
var temp = document.createElement(“div”);
temp.style.cssText = “position:absolute;left:0;top:0;”;
document.body.appendChild(temp);
arguments.callee.offset = -temp.getBoundingClientRect().top - scrollTop;
document.body.removeChild(temp);
temp = null;
}
var rect = element.getBoundingClientRect();
var offset = arguments.callee.offset;
return {
left: rect.left + offset,
right: rect.right + offset,
top: rect.top + offset,
bottom: rect.bottom + offset
};
} else { //对于不支持getBoundingClientRect的修复
var actualLeft = getElementLeft(element);
var actualTop = getElementTop(element);
return {
left: actualLeft - scrollLeft,
right: actualLeft + element.offsetWidth - scrollLeft,
top: actualTop - scrollTop,
bottom: actualTop + element.offsetHeight - scrollTop
}
}
}
因为使用了arguments.callee，因此在strict mode下用不了