var timeout20 = null;
var timeout50 = null;

onscroll = function(e){
  var scrollPosition = window.scrollY / (window.document.body.scrollHeight - window.innerHeight);
  if(0.2 < scrollPosition && scrollPosition < 0.5){
    clearTimeout(timeout20);
    timeout20 = setTimeout(function(){
      console.log('20%');
    }, 500);
  }
  if(0.5 < scrollPosition && scrollPosition < 1){
    clearTimeout(timeout50);
    timeout50 = setTimeout(function(){
      console.log('50%');
    }, 500);
  }
}