How to get user displayname in SharePoint 2010 and even more in 2013 ECMA Script

ExecuteOrDelayUntilScriptLoaded(init,"sp.js");
var currentUser;

function init(){
 this.clientContext = new SP.ClientContext.get_current();
 this.oWeb = clientContext.get_web();
 currentUser = this.oWeb.get_currentUser();
 this.clientContext.load(currentUser);
 this.clientContext.executeQueryAsync(Function.createDelegate(this,this.onQuerySucceeded), Function.createDelegate(this,this.onQueryFailed));
}
function onQuerySucceeded() {
 document.getElementById("currentUser").innerHTML = currentUser.get_loginName();
}
function onQueryFailed(sender, args) {
 alert('Request failed. \nError: ' + args.get_message() + '\nStackTrace: ' + args.get_stackTrace());
}
</script>
<div>Current Logged-In User:<label id="currentUser"></label></div>