Example of avoiding sequential Callback Hell with Ext JS.

app.js

var hello, world;

fetch('hello.json')
.then(function(response) {
    hello = getMessage(response);
    return fetch('world.json');
})
.then(function(response) {
    world = getMessage(response);
    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;
}