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');