Firebase: Detecting if user exists.This snippet detects if a user ID already exists, in order to do first time user functions.

firebase_firstime_user.js

// Assuming you have included the firebase script tag above this javascript
// For reference here is the tag: <script src="https://cdn.firebase.com/js/client/2.3.2/firebase.js"></script>

// Assumes you have already authenticated the user.

// Setup your firebase reference
var ref = new Firebase('https://your-app.firebaseIO-demo.com/');

// Setup a way to get your userid (Assuming using provided firebase authentication method...)
function getUser(authData) {
    switch(authData.provider) {
        case 'password':
           return authData.password.email;
         case 'twitter':
           return authData.twitter.username;
         case 'facebook':
           return authData.facebook.email;
         case 'google':
           return authData.google.email;
         case 'github':
           return authData.github.username;
    }    
}

// Get authentication data
var authData = ref.getAuth();

// Get your user information
var userid = getUser(authData);

// Call your function to check if they are a first time user (aka exists).
checkForFirstTime(userid);

// Setup what to do with the user information.
function userFirstTimeCallback(userId, exists) {
  if (exists) {
    alert('user ' + userId + ' exists!');
    // Do something here you want to do for non-firstime users...
  } else {
    alert('user ' + userId + ' does not exist!');
    // Do something here you want to do for first time users (Store data in database?)
  }
}

// Tests to see if /users/<userId> exists. 
function checkForFirstTime(userId) {
  usersRef.child('users').child(userId).once('value', function(snapshot) {
    var exists = (snapshot.val() !== null);
    userFirstTimeCallback(userId, exists);
  });
}