How to get the return username using javascript? [duplicate]

All calls to SharePoint in the JavaScript Object Model are Asynchronous. This means that you have to make use of callback functions to get values back successfully. In your code, you have used the userQuerySucceeded callback function to fire when your query succeeds. Now to read the login name, you will have to do it inside the callback function. Try this code:

var username;
siteUrl = 'xxx/xxx';

function getLogonUser() {
    var clientContext = new SP.ClientContext(siteUrl);
    this.website = clientContext.get_web();
    this.currentUser = website.get_currentUser();
    clientContext.load(currentUser);
    clientContext.executeQueryAsync(Function.createDelegate(this, this.userQuerySucceeded), Function.createDelegate(this, this.userQueryFailed));

}

function userQuerySucceeded(sender, args) {
    this.username = currentUser.get_loginName();
    alert('Username:' + this.username);
}

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


function showName(){
    getLogonUser();

}