Send notification to one user with firebase cloud function node - node.js

Question

I am trying to send a notification to a specific user using FCM, but I haven't found a way. I have a mobile app and a node server running.
I want to be able to send a notification when the shipment status changes. I have already a function for it in my server I just have to send the notification to the user. Is it possible to achieve this using nodejs or is there a way to implement it in flutter?
I found this code
    var FCM = require('fcm-node')
var serverKey = require('path/to/privatekey.json') //put the generated private key path here            
var fcm = new FCM(serverKey)     
var message = { //this may vary according to the message type (single recipient, multicast, topic, et cetera)        
to: 'registration_token',         
collapse_key: 'your_collapse_key',                
notification: {            
title: 'Title of your push notification',             
body: 'Body of your push notification'         
},                
data: {  //you can send only notification or only data(or include both)            
my_key: 'my value',            
my_another_key: 'my another value'        
}    
}        
fcm.send(message, function(err, response){        
if (err) {            
console.log("Something has gone wrong!")        
} else {            
console.log("Successfully sent with response: ", response)        
this is a npm package taht lets send a notification but it asks for a registration token.
I hope you can help me. Thanks in advance!

Answer 1

Firebase Cloud Messaging has no concept of a user. Instead it can send messages to:
Specific app instances (so your specific app as installed on one specific device), as identified by an FCM/device token.
A group of app instances/tokens, as defined by your application logic.
A specific topic, to which app instances can then subscribe.
It's up to your application logic to decide how to map from your user ID to one of these options. The most common are to:
Store the device token(s) and your user ID in your own database, then look up the device token(s) for a user when needed and fill them in to the API call you already have.
Use a specific topic for each user ID, subscribe the application to that when it starts, and then send a message to that topic when needed. Note that anyone can subscribe to any topic though, so this approach would allow folks to receive message for anyone whose user ID they know.