//获取滚动条距离顶部位置
function getScrollTop() {
    var scrollTop = 0;
    if (document.documentElement && document.documentElement.scrollTop) {
        scrollTop = document.documentElement.scrollTop;
    } else if (document.body) {
        scrollTop = document.body.scrollTop;
    }
    return scrollTop;
}
//获取当前可视范围的高度
function getClientHeight() {
    var clientHeight = 0;
    if (document.body.clientHeight && document.documentElement.clientHeight) {
        clientHeight = Math.min(document.body.clientHeight, document.documentElement.clientHeight);
    } else {
        clientHeight = Math.max(document.body.clientHeight, document.documentElement.clientHeight);
    }
    return clientHeight;
}
//获取文档完整的高度
function getScrollHeight() {
    return Math.max(document.body.scrollHeight, document.documentElement.scrollHeight);
}
//判断滚动条是否达到底部

getScrollTop()   getClientHeight() == getScrollHeight()

更多专业前端知识，请上 【猿2048】www.mk2048.com