Test task - closure

function combinedFunc(a = 0){
    this.res = a;

}   

combinedFunc.prototype.add = function(b){
        this.res += b;
        return this;
};

combinedFunc.prototype.deduct = function(b){
        this.res -= b;
        return this;
};

combinedFunc.prototype.mult = function(b){
        this.res *= b;
        return this;
};

combinedFunc.prototype.div = function(b){
        this.res /= b;
        return this;
};

combinedFunc.prototype.inc = function(){
        this.res++;
        return this;
};

combinedFunc.prototype.valueOf = function(){
    return this.res;
}; 

var cf = new combinedFunc(0);


console.log(cf.add(6).mult(2).div(3).deduct(2).inc() == 3);