Firebase: Creating data if it doesn't exist. This snippet creates a user only if it doesn't already exist.

firebase_create.js

function go() {
  var userId = prompt('Username?', 'Guest');
  var userData = { name: userId };
  tryCreateUser(userId, userData);
}

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

function userCreated(userId, success) {
  if (!success) {
    alert('user ' + userId + ' already exists!');
  } else {
    alert('Successfully created ' + userId);
  }
}

// Tries to set /users/<userId> to the specified data, but only
// if there's no data there already.
function tryCreateUser(userId, userData) {
  var usersRef = new Firebase(USERS_LOCATION);
  usersRef.child(userId).transaction(function(currentUserData) {
    if (currentUserData === null)
      return userData;
  }, function(error, committed) {
    userCreated(userId, committed);
  });
}

This is a comment

What if the user exists and I want to update the information?

@iamchriswick in case you haven't already figured it out (or someone else has the same question), Firebase.transaction() is used to modify data and the first arg is the update function. That means if you leave out the if (currentUserData === null) conditional and just return userData, it would update existing info.

The link for Firebase.transaction() displays YOU'RE VIEWING THE LEGACY DOCS. THEY ARE DEPRECATED AS OF MAY 18, 2016. Its not exactly clear on what tools replace it..