"Create React App" modified sample

reactApp.js

import React, { Component } from 'react';
import firebase from 'firebase';
import logo from './logo.svg';
import './App.css';

class App extends Component {
  constructor (props) {
    super(props);
    this.state = {
      users: ['Carregando...']
    };
  }
  handleSubmit() {
    const newItem = this.refs.itemText.value;
    this.usersRef.push(newItem);
    this.refs.itemText.value = '';
  }
  componentWillMount() {
    const config = {
      apiKey: "XXXXXXXXXX",
      authDomain: "<seu-projeto>.firebaseapp.com",
      databaseURL: "https://<seu-projeto>.firebaseio.com",
      storageBucket: "<seu-projeto>.appspot.com",
    };
    firebase.initializeApp(config);
    this.usersRef = firebase.database().ref('/');
    this.usersRef.on('value', (data) => {
      data = data.val();
      if(data != null) {
        const usersArray = Object.keys(data).map(key => data[key])
        this.setState({users: usersArray})
      }
    });    
  }
  render() {
    const {users} = this.state;
    return (
      <div className="App">
        <div className="App-header">
          <img src={logo} className="App-logo" alt="logo" />
          <h2>Welcome to React</h2>
        </div>
        <input type="text" ref="itemText" placeholder="Digite um novo item aqui..." />
        <button type="button" onClick={() => this.handleSubmit()}>Adicionar</button>
        <ul>
          {users.map((user, index) => (
            <li style={{listStyleType: 'none'}} key={index}>{user}</li>
          ))}
        </ul>
      </div>
    );
  }
}

export default App;