JavaScript closure

Definition for closure : A closure is the local variables for a function – kept alive after the function has returned.

A nice example to explain closure.

//
// define a function which preserves data (count)
//
var CreateCounter = function(){
   var count, f;
   count = 0;
   f = function(){
      count = count+1;
      return count;
   }
   return f;
}

// create objects which remembers its own counter
var counter1 = CreateCounter();
var counter2 = CreateCounter();
alert(counter1());  // 1
alert(counter1());  // 2
alert(counter1());  // 3

alert(counter2());  // 1
alert(counter2());  // 2
alert(counter2());  // 3

alert(counter1());  // 4
alert(counter1());  // 5
alert(counter2());  // 4

Nice article about closure found at http://www.jibbering.com/faq/faq_notes/closures.html

How to use JavaScript functions as values and parameters?

In javascript function are first class objects. That means it can be used as values. Notice the example below

// function definition
function alertHi(name) {
   alert('hi! '+name);
}

// storing string
var name = 'rajakvk';

// storing number
var age = 26;

// storing function
var sayHi = alertHi;

// now sayHi is also function, you can call it like
sayHi();   // hi

Here is example to use function as parameter

// define a function to call a function twice
// here function f passed as parameter
var callTwice = function(f,x) {
   return f(f(x));
}
var addTen = function(x) {
   return x+10;
}
// here function addTen used as value 
var y = callTwice(addTen, 2);   // 22

How to override the parent property from within child object in object oriented javascript?

The code below pretty much explains itself. Just follow the comments. You can also run the code snippet in the firefox console tab

// Parent class definition
var Human = function(name, age) {
    this.name = name;
    this.age = age;
}

// Child class definition
var Indian = function(name, state){
    if(name) this.name = name;
    this.state = state;
}

// Extending
Indian.prototype = new Human('raja',22);

// Creating instance of child class with own name property
var tamilian = new Indian('kvk','TN');

// Creating another instance of child class without own property
var cbe_ian = new Indian('','TN');

alert(tamilian.name);  // kvk
alert(tamilian.age);   // 22
alert(tamilian.state); // TN
alert(cbe_ian.name);   // raja