index.jsx

class Application extends React.Component {
  constructor() {
    super();

    this.state = {
      currentUser: null,
      joinError: null,
    };

  }

  componentWillMount() {

    var _this = this; // TODO can be avoided?
    /**
     * onAuth fires anytime the auth status of this user changes
     */
     _fbBase.onAuth((authData) => {
       if (!authData) {
        //  console.log('unauthenticated user');
         this.setState({
           user: null,
         });
       return;
      }

      console.log('authData', authData);

      var newUserID = authData.facebook.id;

      // save new user to database
      var newUser = _fbUsers.child(newUserID).update(authData, (error) => { // the fat arrouw here should do the trick?
        if (error) {
          alert("Data could not be saved." + error);
        } else {
          console.log("Data saved successfully.", this);
          this.setState({currentUser: authData}); // TODO can be avoided?
        }
      });
    }
  }
}