ExtJS MessageBox does not block like alert(..) does

ExtJS MessageBox does not seem to block like Javascript alert(..) does. I want to show a popup, and then call and AJAX call, upon which it will close the window.

If I call the show method like this then...

//Alert Box :
var alertBox = Ext.create('Ext.window.MessageBox');
var config = {
    title : 'Title',
    closable: true,
    msg: 'Message',
    buttons: Ext.Msg.OK,
    buttonText: { ok: EML.lang.buttons.ok },
    modal: true
};
alertBox.show(config);


//callback
Ext.Ajax.request({
    url: someURL,
    method: 'POST',
    callback: function (options, success, response) {
        //do some stuff
        self.up('window').destroy();
    }
})

..no popup is shown, however the parent window is closes.

If I use a standard Javascript alert then the alert will block. After clicking the OK button, then the callback is executed after which the window closes.

    //Alert Box :
    alert('asdf')


    //callback
    Ext.Ajax.request({
        url: someURL,
        method: 'POST',
        callback: function (options, success, response) {
            //do some stuff
            self.up('window').destroy();
        }
    })

• why does MessageBox not block?
• what can I do to get around this problem?
• does the MessageBox somehow need to know about the parent window to block?

It does not block because blocks are not supported in custom javascript code. As chrome console tells us,

window.alert
function alert() { [native code] }

and native code can block execution.

In ExtJS, you would write a callback for a message box like this:

//Alert Box :
var alertBox = Ext.create('Ext.window.MessageBox');
var config = {
    title : 'Title',
    closable: true,
    msg: 'Message',
    buttons: Ext.Msg.OK,
    buttonText: { ok: EML.lang.buttons.ok },
    modal: true,
    callback:function(btn) {
        //callback
        Ext.Ajax.request({
            url: someURL,
            method: 'POST',
            callback: function (options, success, response) {
                //do some stuff
                self.up('window').destroy();
            }
        })
    }
};
alertBox.show(config);

If such callbacks are deeply nested, I tend to flatten the callbacks like this:

var store = me.down('grid').getStore();
var callback3 = function(btn) {
    if(btn=="yes") store.sync();
};
var callback2 = function() {
    Ext.Msg.prompt('A','Third', callback3);
};
var callback1 = function() {
    Ext.Msg.alert('A','Second', callback2);
};
Ext.Msg.alert('A','First', callback1);

In newer versions of ExtJS, you can check out Ext.Promise, but not in ExtJS 4.1.