SOURCE

var counter = { val: 39 };
var textBox = document.getElementById("num");

/*TweenMax.to(counter, 6, {
  val: 100, 
  onUpdate: function () {
  	textBox.innerHTML = Math.ceil(counter.val);
  },
  ease: Circ.easeOut
});*/

setAnimNum({
  target: textBox,
  from: 23,
  to: -39,
  update: function(val) {
    textBox.innerHTML = Math.ceil(val);
  }
})

function setAnimNum (_obj) {
	var _from   = { val: _obj.from };
	var _to     = _obj.to;
	var _dur    = _obj.dur||4;
	var _target = _obj.target;
  var _update = _obj.update;
  
	TweenMax.to(_from, _dur, {
		val: _to, 
		onUpdate: function(){
      _obj.update(_from.val)
    },
		ease: Circ.easeOut
	});
}

<h1 id='num'>0</h1>

body {
  background: #333;
}
h1 {
  display: inline-block;
  width: 100%;
  color: #fff;
  font-size: 120px;
  font-weight: 300;
  font-family: impact, 'arial black';
  text-align: center;
}