function FX(){    this.init.apply(this, arguments);  }    FX.prototype.init = function(animationFunction, opts) {    this._animationFunction = animationFunction;    this._duration = opts && opts.duration || 250;    this._interval = opts && opts.interval || 40;    this._tween = opts && opts.tween || function(delta) {           var x = 1 - delta,            elasticity = 0.25,            value = 1 - Math.pow(x, 4) + x * x * Math.sin(delta * delta * Math.PI * elasticity);        return value      };    this._onDone = opts && opts.onDone || function (){      console &&  console.log('onDone');    }  }    FX.prototype.start = function(reverse) {    var self = this;    this._playing = true;    this._startT = new Date().getTime();    this._reverse = reverse;    this._onInterval();    this._intervalID = setInterval(function (){     self._onInterval.call(self);    }, this._interval);  }    FX.prototype.stop = function() {    this._playing = false;    clearInterval(this._intervalID);  }    FX.prototype.isGoing = function() { return this._playing }    FX.prototype._onInterval = function() {    var deltaT = new Date().getTime() - this._startT,       duration = this._duration;    if (deltaT >= duration) {      this.stop();      this._animationFunction(this._tween(this._reverse ? 0 : 1))      if (this._onDone) { this._onDone() }      return    }    var delta = deltaT / duration;    if (this._reverse) { delta = 1 - delta }    this._animationFunction(this._tween(delta))  } var fx= new FX(function (x){   document.getElementById('testBox').style.left = x*300+'px'; }); document.getElementById('testBtn').onclick = function (){     document.getElementById('testBox').style.left='0px'    fx.start(); };

　　

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