Javascript/element.clientHeight

paddingを含み、水平スクロールバーの高さ・ボーダー・マージンを含まない

function s1Func(){
  var s1Div = document.getElementById("s1Div");
  console.log(s1Div.scrollHeight);
}

var s2Btn = document.getElementById("s2Btn");
s2Btn.disabled = true;
var s2Div = document.getElementById("s2Div");
function s2Func(){
  if(this.scrollHeight - this.scrollTop === this.clientHeight){
  	s2Btn.disabled = false;
  }
}