Custom scroll implementation for touch

scroller.js

// Handle touch events

var $win = $(window),
    scroller = new Scroller();

this.$win.on("touchstart", function(e){
  e.preventDefault();
  scroller.onTouchStart(e.originalEvent);
});
this.$win.on("touchmove", function(e){
  e.preventDefault();
  scroller.onTouchMove(e.originalEvent);
});
this.$win.on("touchend", function(e){
  e.preventDefault();
  scroller.onTouchEnd(e.originalEvent);
});


// Scroller class

function Scroller() {
  this.startTouchY = 0;
  this.currentY = 0;
}

Scroller.prototype = {
  onTouchStart: function(e) {
    this.currentY = this.startTouchY = e.touches[0].clientY;
    this.startTime = e.timeStamp;
    this.contentStartOffsetY = e.touches[0].pageY - this.startTouchY;
  },
  onTouchMove: function(e) {
    this.currentY = e.touches[0].clientY;
    this.deltaY = this.currentY - this.startTouchY;
    this.deltaTime = e.timeStamp;
    this.newY = (-this.deltaY) + this.contentStartOffsetY;
    this.animateTo(this.newY);
  },
  onTouchEnd: function(e) {
    distance = Math.abs(this.currentY - this.startTouchY);
    if(distance > 20){
      this.doMomentum();
    } 
  },
  animateTo: function(offsetY) {
    this.contentOffsetY = offsetY;
    this.tween = TweenLite.to(window, 0.01, {scrollTo:{y:offsetY}, ease:Power2.easeOut});
  },
  getEndVelocity: function(){
    var distance = (this.currentY - this.startTouchY) * -1,
        time = this.deltaTime -this.startTime,
        velo = distance/time,
        dir = velo < 0 ? -1 : 1,
        abs = Math.abs(velo),
        velocity = _.map(abs, 0, 2.5, 0.25, 1);
    
    velocity *= dir;
    return velocity;
  },
  doMomentum: function() {
    var velocity = this.getEndVelocity(),
        acceleration = velocity < 0 ? 0.0005 : -0.0005,
        displacement = - (velocity * velocity) / (2 * acceleration),
        time = (- velocity / acceleration) * 0.001,
        newY = this.contentOffsetY + displacement;
    this.contentOffsetY = newY;
    this.tween = TweenMax.to(window, time, {
      scrollTo : { y : newY, autoKill:true }, 
      ease: Expo.easeOut,
      overwrite: 5
    });  
  }
}

// Map function from "_" utility namespace
_ = {
 map: function(value, istart, istop, ostart, ostop) {
    return ostart + (ostop - ostart) * ((value - istart) / (istop - istart));
  }  
}