This code allows to intercept ajax requests in Sencha application. It's good base to emulate all the backend for JavaScript client, make it dynamic, add intellect to it, response with delay, and do not touch application code at all. Just include this and relative code.

Ajax request interceptor for ExtJS

Ext.require(["Ext.data.JsonP", "Ext.Ajax", "Ext.util.DelayedTask", "Localresponse.response"], function(){
 Ext.merge(Ext.Ajax , {
  	request: function(params){
			var serviceId = params.url.match( /\/([^\/\?]+)?\?/ ).pop().split("-").join("_");;

			if (undefined !== Localresponse.responses[serviceId]) {
				var delay = 500;
				if (undefined !== Localresponse.responses[serviceId + "_delay"]) {
					delay = Localresponse.responses[serviceId + "_delay"].call();
				};
				var delayedCallback =  Ext.create('Ext.util.DelayedTask', function() {
					var response =  Localresponse.responses[serviceId].apply(this,[params]);
					Ext.callback(params.callback, params.scope || window, [response, true, {responseText: Ext.encode(response)}]);
				});
				delayedCallback.delay(delay);
			} else {
				alert("can't process ajax responce " + serviceId);
			};

		}
	});
});

in app.js