AnimatedValue Animated Value Javascript

function AnimatedValue(value,isAngle) {
    this.write(value);
    this.isAngle    = isAngle;
    this.halfAngle  = Math.PI;
    this.wholeAngle  = Math.PI * 2;
    this.oneAndHalfAngle  = this.wholeAngle + this.halfAngle;
}

AnimatedValue.prototype = {
    get: function () {
        if (this.timeout) {
            var now = Date.now(),
                end = this.frTime + this.timeout;
            if (now >= end) {
                this.timeout = 0;
                return this.toVal;
            } else {
                if(this.isAngle){
                    var shortest_angle=((((this.toVal - this.frVal) % this.wholeAngle) + this.oneAndHalfAngle) % this.wholeAngle) - this.halfAngle;
                    var progress = shortest_angle * (1 - (end - now) / this.timeout);
                    return progress + this.frVal;
                }
                return this.toVal - (this.toVal - this.frVal) * (end - now) / this.timeout;
            }
        } else {
            return this.toVal;
        }
    },
    set: function (value, timeout) {
        if (value != this.toVal) {
            this.frVal = this.get();
            this.toVal = value;
            this.timeout = timeout;
            this.following = undefined;
            this.frTime = Date.now();
        }
    },
    follow: function (following, timeout) {
        this.frVal = this.get();
        this.following = following;
        this.timeout = timeout;
        this.frTime = Date.now();
    },
    write: function (value) {
        this.frVal = value;
        this.toVal = value;
        this.timeout = 0;
        this.frTime = Date.now(); // so end == now
    }
};