An Ext JS extension is a derived class or a subclass of an existing Ext JS class, which is designed to allow the inclusion of additional features. An Ext JS extension is mostly used to add custom functionalities or modify the behavior of an existing Ext JS class. An Ext JS extension can be as basic as the preconfigured Ext JS classes, which basically supply a set of default values to an existing class configuration. This type of extension is really helpful in situations where the required functionality is repeated at several places. Let us assume we have an application where several Ext JS windows have the same help button at the bottom bar. So we can create an extension of the Ext JS window, where we can add this help button and can use this extension window without providing the repeated code for the button. The advantage is that we can easily maintain the code for the help button in one place and can get the change in all places.