counter.js

function Counter () {
    var count = 0;

    function logNewValue() {
        console.log("Count is " + count);
    }

    return {
        add: function () {
            count += 1;
        },
        decrease: function () {
            count -= 1;
        },
        value: function () {
            logNewValue();
        }
    }
}

var c1 = new Counter();
var c2 = new Counter();

c1.add();
c1.add();
c1.add();
c1.decrease()
c1.value(); // Count is 2
console.log(c1.count); // undefined
console.log(typeof c1.logNewValue); // undefined

c2.add();
c2.decrease();
c2.decrease();
c2.value(); // Count is -1
console.log(c2.count); // undefined
console.log(typeof c2.logNewValue); // undefined