script.js

// Initialize the Firebase app by passing in the messagingSenderId
var config = {
  messagingSenderId: "your_messaging_sender_id"
};
firebase.initializeApp(config);
const messaging = firebase.messaging();

// Request for permission
messaging.requestPermission()
.then(function() {
  console.log('Notification permission granted.');
  // TODO(developer): Retrieve an Instance ID token for use with FCM.
  messaging.getToken()
  .then(function(currentToken) {
    if (currentToken) {
      console.log('Token: ' + currentToken)
      sendTokenToServer(currentToken);
    } else {
      console.log('No Instance ID token available. Request permission to generate one.');
      setTokenSentToServer(false);
    }
  })
  .catch(function(err) {
    console.log('An error occurred while retrieving token. ', err);
    setTokenSentToServer(false);
  });
})
.catch(function(err) {
  console.log('Unable to get permission to notify.', err);
});

// Handle incoming messages
messaging.onMessage(function(payload) {
  console.log("Notification received: ", payload);
  toastr["info"](payload.notification.body, payload.notification.title);
});

// Callback fired if Instance ID token is updated.
messaging.onTokenRefresh(function() {
  messaging.getToken()
  .then(function(refreshedToken) {
    console.log('Token refreshed.');
    // Indicate that the new Instance ID token has not yet been sent 
    // to the app server.
    setTokenSentToServer(false);
    // Send Instance ID token to app server.
    sendTokenToServer(refreshedToken);
  })
  .catch(function(err) {
    console.log('Unable to retrieve refreshed token ', err);
  });
});

// Send the Instance ID token your application server, so that it can:
// - send messages back to this app
// - subscribe/unsubscribe the token from topics
function sendTokenToServer(currentToken) {
  if (!isTokenSentToServer()) {
    console.log('Sending token to server...');
    // TODO(developer): Send the current token to your server.
    setTokenSentToServer(true);
  } else {
    console.log('Token already sent to server so won\'t send it again ' +
        'unless it changes');
  }
}

function isTokenSentToServer() {
  return window.localStorage.getItem('sentToServer') == 1;
}

function setTokenSentToServer(sent) {
  window.localStorage.setItem('sentToServer', sent ? 1 : 0);
}