firebase init

import * as firebase from 'firebase';
const firebaseConfig = {
  apiKey: "AIzaSyAfHOFdjPwByt_S7RSPcyF5OPzwp_????",
  authDomain: "??????-44ce2.firebaseapp.com",
  databaseURL: "https://??????-44ce2.firebaseio.com",
  storageBucket: "",
};
const app = firebase.initializeApp(firebaseConfig);

using realtime firebase db

var user = firebase.auth().currentUser;
  if (user != null) {
    this.itemsRef = this.getRef().child('Stores/'+user.uid+'/');
  }else {
    alert('you have to login again!');
  };

  const connectedRef = firebase.database().ref('.info/connected');
  connectedRef.on('value', snap => {
    if (snap.val() === true) {
      this.listenForItems(this.itemsRef);
    } else {
      this.offlineForItems();
    }
});