Basic counter that demonstrates javascript closure concept

counter.js

// In the function below changeBy is a private function
// whereas increment and decrement are public and hence
// can be invoked outside the Counter's scope

function Counter(initialCounter) {
  var privateCounter = initialCounter || 0;
  function changeBy(val) {
    privateCounter += val;
  }
  return {
    increment: function() {
      changeBy(1);
    },
    decrement: function() {
      changeBy(-1);
    },
    value: function() {
      return privateCounter;
    }
  };   
};

var counter = new Counter(1);

console.log(counter.value()); // logs 1
counter.increment();
counter.increment();
console.log(counter.value()); // logs 3
counter.decrement();
console.log(counter.value()); // logs 2

// should give an error as it is out of scope
// console.log(counter.changeBy(1));