让div运动起来 js实现缓动效果

var tween = {  linear:function(t,b,c,d){    return c*t/d + b;  },  easeIn:function(t,b,c,d){    return c * ( t /= d ) * t + b;  },  strongEaseIn:function(t,b,c,d){    return c * ( t /= d ) * t * t * t * t + b;  },  strongEaseOut:function(t,b,c,d){    return c * ( ( t = t / d -1 ) * t * t * t * t +1 ) + b;  },  sineaseIn:function(t,b,c,d){    return c * ( t /= d ) * t * t + b;    },  sineaseOut:function(t,b,c,d){    return c * ( ( t = t / d -1 ) * t * t *t +1 ) + b;  }};var Animate = function(dom){  this.dom = dom;  this.startTime = 0;  this.startPos = 0;  this.endPos = 0;  this.propertyName = null;  this.easing = null;  this.duration = null;}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,'strongEaseOut');