JS判断滚动条是否到底

function scroll() {
  var scrollTop = getScrollTop();
  var scrollLeft= document.documentElement.scrollLeft;

  var a = scrollTop == 0 ? document.body.clientHeight : document.documentElement.clientHeight;
  var b = scrollTop == 0 ? document.body.scrollTop : document.documentElement.scrollTop;
  var c = scrollTop == 0 ? document.body.scrollHeight : document.documentElement.scrollHeight;

  alert(a+"_"+b +"_"+c+"  "+scrollLeft);
 }
 function getScrollTop() {
  var s = ('pageYOffset' in window) ? window.pageYOffset
   : document.compatMode === "BackCompat"
   && document.body.scrollTop
   || document.documentElement.scrollTop ;
   return s;
 }

a+b==c