A proposal for an ExtJS 3 component initialisation pattern.

foo.ext3.js

Ext.namespace('foo');

foo.UpdatePositionDialog = Ext.extend(Ext.Window, {

    /**
     * @param {Object} config Instance configuration options
     */
    constructor:function (config) {

        // Merge the incoming config with the defaults. Externally provided properties take precedence.
        config = Ext.apply({

            cls:'UpdatePositionDialog',

            width:600,

            height:'auto'
            
        },
        config || {}); // Alas Ext is still a bit rubbish so we have to guard against a null config

        foo.UpdatePositionDialog.superclass.constructor.call(this, config);
    }

});