The Ext.Ajax class is one instance of the Ext.data.Connection class, so it cannot be overridden in this way.
You can replace the singleton function, rather than create a “correct” override.
One way would be to do the following:
Ext.Ajax.request = Ext.Function.createInterceptor(Ext.Ajax.request, function(){
console.log('override');
});
, , .
: sencha touch override ext.ajax