Test if a data exist in Firebase

HTML

<script src="http://static.firebase.com/v0/firebase.js"></script>
Example for <a href='http://stackoverflow.com/q/24824732/209103'>Test if a data exist in Firebase</a><br/>
<button id='init'>Create user...</button>
<div id='log'></div>

JavaScript

var ref = new Firebase("https://stackoverflow.firebaseio.com/24824732/");
var users = ref.child("users");

var log = document.getElementById('log');

document.getElementById('init').addEventListener('click', function(event) {
    var username = prompt('What user do you want to create?');
    //users.child(username).set({ name: username });
    users.once('value', function(snapshot) {
        if (!snapshot.hasChild(username)) {
            users.child(username).set({ name: username });
        }
        else {
            alert("That user already exists");
        }
    });
});

users.on('child_added', function(snapshot) {
    var val = snapshot.val();
    var pre = document.createElement('pre');
    pre.id = val.name;
    pre.innerText = JSON.stringify(val);
    log.appendChild(pre);
});

users.on('child_changed', function(snapshot) {
    var val = snapshot.val();
    var pre = document.getElementById(val.name);
    pre.innerText = JSON.stringify(val);
});