Example of avoiding Callback Hell with Ext JS.

app.js

Ext.Deferred.all([fetch('hello.json'), fetch('world.json')])
.then(function(responses) {
    var hello = getMessage(responses[0]),
        world = getMessage(responses[1]);
    Ext.Msg.alert('Success', hello + world);
})
.otherwise(function(response) {
    var error = getMessage(response);
    Ext.Msg.alert('Failure', error);
});

function fetch(url) {
    var deferred = new Ext.Deferred();

    Ext.Ajax.request({
        url: url,
        success: function(response, opts) {
            deferred.resolve(response);
        },
        failure: function(response, opts) {
            deferred.reject(response);
        }
    });

    return deferred.promise;
}

function getMessage(response) {
    var json = response.responseText;
    return Ext.JSON.decode(json).message;
}