I wanna move my button to bottom with a couple of codes here :

var y = [];
for(var i = 0;i < 8;i++){
   y[i] = 0;
   interval[i] = setInterval(function(){
      y[i]++;
      if(parseInt(y[i]) >= window.innerHeight){
          clearInterval(this);
      }
      console.log(y[i]);
      thisbutton.style.top = y[i] + "px";
   },10);
}

My problem is that y[i] is NaN.
Can you tell me why?
thanks