Tweener.js

var easingTypes = require('tween-functions');

module.exports = function(config) {
  return new Tween(config)
}

function Tween(config){
  this._rafLoop = this._rafLoop.bind(this)
  this.terminate = this.terminate.bind(this)

  this._t0 = Date.now()
  this._config = config
  this._rafLoop()
}

Tween.prototype._rafLoop = function() {
  if(this._break){ return }

  var {duration, start, end, easingType} = this._config
  var now = Date.now()
  var elapsed = now - this._t0

  if(elapsed >= duration){
    this._config.onFrame(end)
    this._config.onEnd()
    return
  }

  var tweenVal = easingTypes[easingType](elapsed, start, end, duration)
  this._config.onFrame(tweenVal)

  requestAnimationFrame(this._rafLoop)
}

Tween.prototype.terminate = function(){
  this._break = true
}