Meteor : wait for a callback - asynchronous

Question

WHen I create an account on my Meteor app, I check in several time the validity of each element. Then I create the user. If no error appeared, I show a success div, and an error one on the other case.
Accounts.createUser(options ,function(err){
if(err){
console.log("rorororororroro");
if(err.message == "Email already exists. [403]"){
Session.set('error', regAlert("This email adress already exists", tmpl));
}else{
Session.set('error', regAlert("Sorry, but an unidentified error occured. Please retry later. If the error persists, contact the support"));
}
}else if(Meteor.call('talknameIsAvailable', talkname)){
Session.set('error', regAlert("A user with this talkname already exists", tmpl));
}
});
if(!Session.get('error')){
$('#register-success').show("slow");
//console.log("[SUCCESS] Account created");
//document.location.href="/";
}else{
$('#register-error').show("slow");
}
The problem is that I always show the success div, as the last if / else is computed before the callback set the error session var when the email is already used.
What's the best way in Meteor to handle this case ?
Thanks you

Answer 1

When Session.get("error") is undefined, (!Session.get("error")) returns true.
try:
if (Session.get("error") !== void 0) {
}

Answer 2

I added .wait() at the end of the createUser function :
Accounts.createUser(options ,function(err){
if(err){
console.log("rorororororroro");
if(err.message == "Email already exists. [403]"){
Session.set('error', regAlert("This email adress already exists", tmpl));
}else{
Session.set('error', regAlert("Sorry, but an unidentified error occured. Please retry later. If the error persists, contact the support"));
}
}else if(Meteor.call('talknameIsAvailable', talkname)){
Session.set('error', regAlert("A user with this talkname already exists", tmpl));
}
}).wait();