From Learn About the Ext JavaScript Library

Javascript is different from other object-oriented languages like C++, Java or PHP. It's not based on the concept of classes, but it uses a concept named prototype. It is useful to note that the term class appears often even though it isn't exactly correct. When you see class think object.

Contents 1 Object creation 2 Using a Constructor Function 2.1 Screen Shot of FireFox + Firebug 3 Share methods

Object creation

Making an object in Javascript is very simple :

var myObject = {
  aVar: 15,
  aMethod: function() {
    alert("I'm a method of the object myObject." + "aVar: " + this.aVar);
  }
}

You don't have to define a class and instantiate it to create an object. We are using here an object initializer. It's perfect in the case of a single object. If we need multiple objects with the same type, then we have to use a constructor function and the new keyword.

Using a Constructor Function

There are no classes in Javascript, but constructors exist. You write a function and then you create an object with the new keyword.

// At first, we write an empty constructor for our class
function myClass() {
  this.aVar = 15;
  this.aMethod = function() {
    alert("I'm a method of the object myObject.");
  }
}
 
// We make an instance of our class
var A = new myClass();
 
// Display 15
alert(A.aVar);
 
// 2nd instance
var B = new myClass();

Screen Shot of FireFox + Firebug

Click on Run to see the alert panel.

Share methods

All instances of any class share a single prototype object. This is where functions and shared values should be placed:

// We define a method of the prototype object
Ext.apply(myClass.prototype, {
 
    defaultClassName: "x-widget-class",
 
    sharedMethod: function() { alert("I'm a shared method") }
});
 
//  Display our message
A.sharedMethod();
 
// Same message
B.sharedMethod();

There is no method named sharedMethod in the myClass definition. Javascript looks for a method with this name in the prototype object of myClass and calls it if it exists.