关键API：
Element.getBoundingClientRect()
mdn：https://developer.mozilla.org/en-US/docs/Web/API/Element/getBoundingClientRect
用法：
var rect = element.getBoundingClientRect(); console.log(rect.top, rect.right, rect.bottom, rect.left);
获取相对位置：
var bodyRect = document.body.getBoundingClientRect(), elemRect = element.getBoundingClientRect(), offset = elemRect.top - bodyRect.top; alert('Element is ' + offset + ' vertical pixels from <body>');
参考网址：http://stackoverflow.com/questions/442404/retrieve-the-position-x-y-of-an-html-element
我只是个搬运工，一起学习了~