判断浏览器 是否到达最低端

window.onscroll  = function (){
    var marginBot = 0;
    if (document.compatMode === "CSS1Compat"){
        marginBot = document.documentElement.scrollHeight - (document.documentElement.scrollTop+document.body.scrollTop)-  document.documentElement.clientHeight;
    } else {
        marginBot = document.body.scrollHeight - document.body.scrollTop-  document.body.clientHeight;
    }
    if(marginBot<=0) {
        //do something
    }
}