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Code in this post can be obsolete, however, principles and theory may still apply.


If you have not already done so, study Writing a Big Application in Ext (Part 1) and Writing a Big Application in Ext (Part 2)before you read this article. It would be very hard, if not impossible, to understand concepts explained here before you fully understand the first and second part.


Helping on the forum and reading code of others that failed to extend Ext classes, revealed more errors that users, especially beginners, commonly make. Therefore, I’ve decided to start this article that will collect these errors and will explain why the errors are errors. I mean it as loosely ended as I may discover more errors and ways of avoiding them so I plan just to add them to this article, not endlessly create parts 4, 5, etc…

… continued: Most Common Sources of Troubles

Here we go:
  1. Unnecessary Extending
  2. Adding Objects to Prototype
  3. Hard Coding ids

Unnecessary Extending

The main reasons for extending are:
  • re-usability
  • adding functionality
  • combination of them
so we extend if we need a re-usable component or we need to add a functionality (new methods) or both. If we are after re-usability the extension can be as simple as:
MyPortlet = Ext.extend(Ext.Panel, {
You see what happens? We are going to use MyPortlet many times so instead of scatter the above configuration in 10,000 lines application code 100 times, we create this simple extension and we save 297 lines of code. The other aspect is that if we upgrade our ‘mygraph’ to ‘mygraph_new’ the only place where to change it is our extension saving us searching out code for all occurrences of ‘mygraph’ (100 times) and replacing it with ‘mygraph_new’ 100 times. (Well, 100 is exaggerated, but you get the point, right?) If we are after adding functionality, which can be also simple, we add some “logic”:
MyPanel = Ext.extend(Ext.Panel, {
    onRender:function() {
        MyPanel.superclass.onRender.apply(this, arguments);
Here we add some logic to Panel, it does more that it did before. There is no need to extend in all other cases.  

Adding Objects to Prototype

Run this code first:
Extending Error: Object in prototype

  Ext.BLANK_IMAGE_URL = 'ext/resources/images/default/s.gif';
  Ext.onReady(function() {
    MyPanel = Ext.extend(Ext.Panel, {
        ,panelConfig: {

        ,initComponent:function() {
            var config = {
                bodyStyle:'background-color:' + this.panelConfig.bodyBg
            }; // eo config object

            // apply config
            Ext.apply(this, Ext.apply(this.initialConfig, config));

            MyPanel.superclass.initComponent.apply(this, arguments);
        } // eo function initComponent

        ,applyBackground:function(color) {
            this.panelConfig.bodyBg = color;
        } // eo function applyBackground

    }); // eo extend

    var p1 = new MyPanel({
         title:'Panel with Blue Background'


    var p2 = new MyPanel({
         title:'Panel with Red Background'

What do we expect? It is written in titles of panels: Top panel (p1) should have blue body background because we set it to it after it is created. And bottom panel (p2) should have red because we just create default MyPanel. But it is blue too!!! Why? The reason is simple: panelConfig is object that is created during class definition and it is added to MyPanel prototype. Objects (arrays too) are accessed by reference so p1 and p2 share the same instance of panelConfig. Setting bodyBg property in applyBackground method changes this single instance of panelConfig object. So we create p2 with blue background too. You see, here it is clearly and immediately visible that something went wrong but making this error can lead to weeks of wasted debugging time in real applications. Imagine you have a store in prototype…  

Hard Coding ids

Very simple, but deadly mistake is to set ids in the extension either to the main extension object or on its items, toolbars, buttons, etc. If a hard coded ids are set we cannot create two or more instances of our extension, can we?

Loose End

That’s all for now but if I discover more errors I will add them above. Stay tuned! Do not forget to read Part 1 and Part 2 of this article. Follow up: Factory Functions in Ext Extensions (Abstract Classes)
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