Class System

This guide is intended for any developer who wants to create new or extend existing classes in Ext JS 4.x and 5.x.


Ext JS 5 ships with more than 300 classes. We have more than 2 million developers to date and they coming from various backgrounds and locations. At that scale, we face a big challenge in providing a common code architecture that is:

  • Familiar and simple to learn
  • Fast to develop, easy to debug, and painless to deploy
  • Organized, extensible, and maintainable

Because JavaScript is a classless, prototype-oriented language, one of its most powerful features is flexibility. There are multiple solutions to any problem using many different coding styles and techniques. However, this comes with the cost of being unpredictable. Without a unified structure, JavaScript code can be difficult to understand, maintain, and re-use.

Class-based programming, on the other hand, is still the most popular model of Object Oriented Programming. Class-based languages usually requires strong-typing, encapsulation, and standard coding conventions. By making developers adhere to a large set of principles, code is more likely to be predictable, extensible, and scalable over time. However, this model doesn’t have JavaScript’s dynamic capability.

Each approach has pros and cons, but can we keep the good parts of both while hiding the bad parts? The answer is yes, and you can find the solution in Ext JS.

Naming Conventions

Using consistent naming conventions throughout your code base for classes, namespaces and filenames helps keep your code organized, structured and readable.


Class names may only contain alphanumeric characters. Numbers are permitted but discouraged, unless they belong to a technical term. Do not use underscores, hyphens, or any other non-alphanumeric character. For example:

  • MyCompany.useful_util.Debug_Toolbar is discouraged
  • MyCompany.util.Base64 is acceptable

Class names should be grouped into packages when appropriate and properly namespaced using object property dot-notation (.). At a minimum, there should be one unique top-level namespace followed by the class name. For example:

The top-level namespaces and the actual class names should be CamelCased. Everything else should be all lower-cased. For example:


Classes that are not distributed by Sencha should never use Ext as the top-level namespace.

Acronyms should also follow CamelCased convention listed above. For example:

  • instead of
  • MyCompany.util.HtmlParser instead of MyCompary.parser.HTMLParser
  • MyCompany.server.Http instead of MyCompany.server.HTTP

Source Files

The names of the classes map directly to the file paths in which they are stored. As a result, there must only be one class per file. For example:

  • Ext.util.Observable is stored in path/to/src/Ext/util/Observable.js
  • Ext.form.action.Submit is stored in path/to/src/Ext/form/action/Submit.js
  • MyCompany.chart.axis.Numeric is stored in path/to/src/MyCompany/chart/axis/Numeric.js

path/to/src is the directory of your application’s classes. All classes should stay under this common root and should be properly namespaced for the best development, maintenance, and deployment experience.

Methods and Variables

  • In a similar fashion to class names, method and variable names may only contain alphanumeric characters. Numbers are permitted but discouraged unless they belong to a technical term. Do not use underscores, hyphens, or any other non-alphanumeric character.

  • Method and variable names should always be in camelCased. This also applies to acronyms.


  • Acceptable method names:

    • encodeUsingMd5()
    • getHtml() instead of getHTML()
    • getJsonResponse() instead of getJSONResponse()
    • parseXmlContent() instead of parseXMLContent()
  • Acceptable variable names:

    • var isGoodName
    • var base64Encoder
    • var xmlReader
    • var httpServer


  • Class property names follow the exact same convention except when they are static constants.

  • Static class properties that are constants should be all upper-cased. For example:

    • Ext.MessageBox.YES = "Yes"
    • Ext.MessageBox.NO = "No"
    • MyCompany.alien.Math.PI = "4.13"


The Old Way

If you have ever used any version of Ext JS previous to Ext JS 4, you are certainly familiar with Ext.extend to create a class:

var MyWindow = Ext.extend(Object, { ... });

This approach is easy to follow in order to create a new class that inherits from another. Other than direct inheritance, we didn’t have a fluent API for other aspects of class creation. This left out things such as configuration, statics, and mixins. We will be reviewing these items in detail later in this guide.

Let’s take a look at another example: = Ext.extend(Ext.Window, { ... });

In this example, we want to namespace our new class, and make it extend from Ext.Window. There are two concerns we need to address:

  1. needs to be an existing object before we can assign Window as its property
  2. Ext.Window needs to exist / loaded on the page before it can be referenced

The first item is usually solved with Ext.namespace (aliased by Ext.ns). This method recursively transverses through the object / property tree and creates them if they don’t exist. This necessitates adding them above Ext.extend.

Ext.ns(''); = Ext.extend(Ext.Window, { ... });

The second issue is not easy to address because Ext.Window might depend on many other classes. In turn, these dependencies may depend on the existance of other classes. For that reason, applications written prior to Ext JS 4 included the whole library in the form of ext-all.js, regardless of whether the application may only need a small portion of the framework.

The New Way

Ext JS 4 eliminated these drawbacks with a single method you need to remember for class creation: Ext.define. Its basic syntax is as follows:

Ext.define(className, members, onClassCreated);
  • className: The class name
  • members is an object that represents a collection of class members in key-value pairs
  • onClassCreated is an optional function callback that is invoked when all dependencies of the defined class are ready and the class itself is fully created. Due to the asynchronous nature of class creation, this callback can be useful in many situations. These will be discussed further in Section IV


Ext.define('My.sample.Person', {
    name: 'Unknown',

    constructor: function(name) {
        if (name) {
   = name;

    eat: function(foodType) {
        alert( + " is eating: " + foodType);

var aaron = Ext.create('My.sample.Person', 'Aaron');"Salad"); // alert("Aaron is eating: Salad");

Note: We created a new instance of My.sample.Person using the Ext.create() method. We could have used the new keyword (new My.sample.Person()). However it is recommended to get in the habit of always using Ext.create since it allows you to take advantage of dynamic loading. For more info on dynamic loading see the Getting Started guide


In Ext JS 4, we introduced a dedicated config property that gets processed by the powerful Ext.Class pre-processors before the class is created. Features include:

  • Configurations are completely encapsulated from other class members
  • Getter and setter methods for every config property are automatically generated into the class prototype during class creation if methods are not already defined.
  • An apply method is also generated for every config property. The auto-generated setter method calls the apply method internally before setting the value. You may override the apply method for a config property if you need to run custom logic before setting the value. If apply does not return a value, the setter will not set the value.

For Ext classes that use the configs, Ext JS 5 eliminated the need to call initConfig() manually. However, for your own classes that extend Ext.Base, initConfig() still needs to be called.

You can see configuration examples below.

Ext.define('My.own.Window', {
   /** @readonly */
   isWindow: true,

   config: {
       title: 'Title Here',

       bottomBar: {
           height: 50,
           resizable: false

   applyTitle: function(title) {
       if (!Ext.isString(title) || title.length === 0) {
           alert('Error: Title must be a valid non-empty string');
       else {
           return title;

   applyBottomBar: function(bottomBar) {
       if (bottomBar) {
           if (!this.bottomBar) {
               return Ext.create('My.own.WindowBottomBar', bottomBar);
           else {

/** A child component to complete the example. */
Ext.define('My.own.WindowBottomBar', {
   config: {
       height: undefined,
       resizable: true

And here’s an example of how it can be used:

var myWindow = Ext.create('My.own.Window', {
    title: 'Hello World',
    bottomBar: {
        height: 60

alert(myWindow.getTitle()); // alerts "Hello World"

myWindow.setTitle('Something New');

alert(myWindow.getTitle()); // alerts "Something New"

myWindow.setTitle(null); // alerts "Error: Title must be a valid non-empty string"

myWindow.setBottomBar({ height: 100 });

alert(myWindow.getBottomBar().getHeight()); // alerts 100


Static members can be defined using the statics config

Ext.define('Computer', {
    statics: {
        instanceCount: 0,
        factory: function(brand) {
            // 'this' in static methods refer to the class itself
            return new this({brand: brand});

    config: {
        brand: null

var dellComputer = Computer.factory('Dell');
var appleComputer = Computer.factory('Mac');

alert(appleComputer.getBrand()); // using the auto-generated getter to get the value of a config property. Alerts "Mac"

Errors Handling & Debugging

Ext JS includes some useful features that will help you with debugging and error handling.

  • You can use Ext.getDisplayName() to get the display name of any method. This is especially useful for throwing errors that have the class name and method name in their description:

    throw new Error('['+ Ext.getDisplayName(arguments.callee) +'] Some message here');
  • When an error is thrown in any method of any class defined using Ext.define(), you should see the method and class names in the call stack if you are using a WebKit based browser (Chrome or Safari). For example, here is what it would look like in Chrome:

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