Edit in JSFiddle

var Counter = function (counter) {
		this.counter = counter;				
    this.increment = function (b) {
        this.counter += b;
    };
};

var Counter2 = function (counter) {
		this.counter = counter;
		};
Counter2.prototype.increment = function (b) {
    this.counter += b;
};

var counter_0 = new Counter(0);
var counter_1 = new Counter(1);

var counter2_0 = new Counter2(0);
var counter2_1 = new Counter2(1);

counter_0.increment = function (b) {
	this.counter += b*2;
}

counter_0.increment(2);
counter_1.increment(2);

console.log("Counter 0: " + counter_0.counter);
console.log("Counter 1: " + counter_1.counter);

Counter2.prototype.increment = function (b) {
    this.counter += b*2;
};

counter2_0.increment(2);
counter2_1.increment(2);

console.log("Counter2 0: " +  counter2_0.counter);
console.log("Counter2 1: " + counter2_1.counter);