Need some help with JavaScript variables in SharePoint 2010

Can you say why this code doesn't work for me? SharePoint 2010. I have no ideas any more about how to set this value to global object :(

ExecuteOrDelayUntilScriptLoaded(getWebUserData, "sp.js");
var context = null;
var web = null;
var currentUser = null;
var currentUserData = {
"userLogin": "templogin"
};

function getWebUserData() {
    context = new SP.ClientContext.get_current();
    web = context.get_web();
    currentUser = web.get_currentUser();
    currentUser.retrieve();
    context.load(web);
    context.executeQueryAsync(Function.createDelegate(this, this.onSuccessMethod), 
         Function.createDelegate(this, this.onFailureMethod));
};

function onSuccessMethod(sender, args) {
    var userObject = web.get_currentUser();
    alert('User name:' + userObject.get_title() + '\n Login Name:' + 
             userObject.get_loginName());
    // export loginName value into global object
    currentUserData.userLogin = userObject.get_loginName();
};

function onFailureMethod(sender, args) {
    alert('request failed ' + args.get_message() + '\n' + args.get_stackTrace());
};

// output (alert) value from global obj properties.
// it doesn's work.
alert("Once more time: \n Current User Login: " + currentUserData.userLogin);