How do I detect if a user is already logged in Firebase?

firebase_auth_sample

  var config = {
    apiKey: "....",
    authDomain: "...",
    databaseURL: "...",
    projectId: "..",
    storageBucket: "..",
    messagingSenderId: ".."
  };
  firebase.initializeApp(config);

  $( document ).ready(function() {
        console.log( "testing.." );
        var user = firebase.auth().currentUser;
        console.log(user);
  });