Animate.prototype.start = function(propertyName,endPos,duration,easing){
this.startTime = +new Date;
this.startPos = this.dom.getBoundingClientRect()[propertyName];
this.propertyName = propertyName;
this.endPos = endPos;
this.duration = duration;
this.easing = tween[easing];

var self = this;
var timeId = setInterval(function(){
if(self.step() === false){
clearInterval(timeId);
}
},19);
}

Animate.prototype.step = function(){
var t = +new Date;
if(t>=this.startTime + this.duration){
this.update(this.endPos);
return false;
}
var pos = this.easing(t-this.startTime,this.startPos,this.endPos - this.startPos,this.duration);
this.update(pos);
}

Animate.prototype.update = function(pos){
this.dom.style[this.propertyName] = pos + 'px';
}

var div = document.getElementById('div');
var animate = new Animate(div);
animate.start('left',500,1000,'h3EaSEOut');