How to use

subscribeToTopic

function

in

Module

messaging()
   .getToken()
   .then(fcmToken => {
    if (fcmToken) {
     onRegister(fcmToken);
     messaging().subscribeToTopic('all');
    } else {
     console.log('[FCMService] user does not have a device token');
    }
   })
   .catch(error => {
    console.log('[FCMService] getToken rejected ', error);
   });