How busy is the CPU?

Ok, I officially am a JS noob...thanks for the explanation. At first I did not see the difference, but I saw what to check.
I kind'a get it now and this helped:

var glow=function(led, stp) {
  var intensity=0;
  var dir=1.0;
  var this_led=led;
  var this_stp=stp;
  function intensity_step() {
    intensity+=dir*this_stp;
    if (intensity>=1.0) {
      intensity=1.0;
      dir=-dir;
    }
    if (intensity<=0.0) {
      intensity=0;
      dir=-dir;
    }
    analogWrite(this_led, intensity);
  }
  return intensity_step;
};

var led1=glow(D13,0.01);
setInterval(led1, 2);

Is that behavior a generic JS thing or the implementation of setInterval()?