Example of Callback Hell with Ext JS.

app.js

Ext.Ajax.request({
    url: 'hello.json',
    success: function(response, opts) {
        var hello = getMessage(response);
        Ext.Ajax.request({
            url: 'world.json',
            success: function(response, opts) {
                var world = getMessage(response);
                Ext.Msg.alert('Success', hello + world);
            },
            failure: function(response, opts) {
                var error = getMessage(response);
                Ext.Msg.alert('Failure', error);
            }
        });
    },
    failure: function(response, opts) {
        var error = getMessage(response);
        Ext.Msg.alert('Failure', error);
    }
});

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