SmarterKey

function go() {
  var userId = prompt('Username?', 'Guest');
  checkIfUserExists(userId);
}

var USERS_LOCATION = 'https://SampleChat.firebaseIO-demo.com/users';

function userExistsCallback(userId, exists) {
  if (exists) {
    alert('user ' + userId + ' exists!');

SmarterKey

function makeList(ref) {
  var fruits = ["banana", "apple", "grape", "orange"];
  for (var i = 0; i < fruits.length; i++) {
    ref.push(fruits[i]);
  }
}

function getFirstFromList(ref, cb) {
  ref.startAt().limit(1).once("child_added", function(snapshot) {
    cb(snapshot.val());