Best Practices concerning initComponent() in Ext.define()

Question

I\'m writing all my components in ExtJS\'s new MVC fashion using Ext.define().

I struggle a bit whether define properties inside of initComponent(

Answer 1

I want to add to Lionel's answer that it is better to declare any non-primitive config in initComponent. (By primitive I mean string, boolean and number). Array and Object go into initComponent.
So definition should look like this:

Ext.define('My.NewClass', {
  extend: 'OldClass',
  // here all primitive configs:
  cls: 'x-my-cls',
  collapsible: true,
  region: 'west',
  // and so on ...

  initComponent: function() {
    // here you declare non-primitive configs:
    this.tbar = [/* blah-blah */];
    this.columns = [/* blah-blah */];
    this.viewConfig = {/* blah-blah */};
    // and so on ...

    this.callParent(arguments);
  }

  // other stuff
}

The reason why you should put all non-primitive configs into initComponent is that otherwise configs of all instances will refer to the same objects. For example if you define NewClass like:

Ext.define('My.NewClass', {
  extend: 'OldClass',
  bbar: Ext.create('Ext.toolbar.Toolbar', {
  // ...

bbars of all instances will refer to the same object. And therefore every time you create new instance bbar disappears from the preveous instance.