Javascript/element.scrollHeight

画面上に表示されていないコンテンツを含む要素の内容の高さ

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;
  }
}