Node Server code to use Firebase Admin SDK to send out push notifications. When a post endpoint is hit on the Node JS server, it delivers a notification to a Flutter app using Firebase Messaging.

index.js

import {initializeApp, applicationDefault } from 'firebase-admin/app';
import { getMessaging } from "firebase-admin/messaging";
import express, { json } from "express";
import cors from "cors";


process.env.GOOGLE_APPLICATION_CREDENTIALS;

const app = express();
app.use(express.json());

app.use(
  cors({
    origin: "*",
  })
);

app.use(
  cors({
    methods: ["GET", "POST", "DELETE", "UPDATE", "PUT", "PATCH"],
  })
);

app.use(function(req, res, next) {
  res.setHeader("Content-Type", "application/json");
  next();
});


initializeApp({
  credential: applicationDefault(),
  projectId: 'potion-for-creators',
});

app.post("/send", function (req, res) {
  const receivedToken = req.body.fcmToken;
  
  const message = {
    notification: {
      title: "Notif",
      body: 'This is a Test Notification'
    },
    token: "YOUR FCM TOKEN HERE",
  };
  
  getMessaging()
    .send(message)
    .then((response) => {
      res.status(200).json({
        message: "Successfully sent message",
        token: receivedToken,
      });
      console.log("Successfully sent message:", response);
    })
    .catch((error) => {
      res.status(400);
      res.send(error);
      console.log("Error sending message:", error);
    });
  
  
});

app.listen(3000, function () {
  console.log("Server started on port 3000");
});

Hi everyone:

I am getting an error:
Error: Exactly one of topic, token or condition is required
at validateMessage (/opt/render/project/src/node_modules/firebase-admin/lib/messaging/messaging-internal.js:53:15)
at Messaging.send (/opt/render/project/src/node_modules/firebase-admin/lib/messaging/messaging.js:185:50)
at file:///opt/render/project/src/index.js:47:6
at Layer.handle [as handle_request] (/opt/render/project/src/node_modules/express/lib/router/layer.js:95:5)
at next (/opt/render/project/src/node_modules/express/lib/router/route.js:144:13)
at Route.dispatch (/opt/render/project/src/node_modules/express/lib/router/route.js:114:3)
at Layer.handle [as handle_request] (/opt/render/project/src/node_modules/express/lib/router/layer.js:95:5)
at /opt/render/project/src/node_modules/express/lib/router/index.js:284:15
at Function.process_params (/opt/render/project/src/node_modules/express/lib/router/index.js:346:12)
at next (/opt/render/project/src/node_modules/express/lib/router/index.js:280:10)

When I tried to chatgpt it, not much useful information was received. This is how my code looks:
` import {initializeApp, applicationDefault } from 'firebase-admin/app';
 import { getMessaging } from "firebase-admin/messaging";
 import express from "express";
 import cors from "cors";


 process.env.GOOGLE_APPLICATION_CREDENTIALS;

 const app = express();
 app.use(express.json());

 app.use(
  cors({
     origin: "*",
         })
     );

app.use(
   cors({
       methods: ["GET", "POST", "DELETE", "UPDATE", "PUT", "PATCH"],
     })
 );

 app.use(function(req, res, next) {
 res.setHeader("Content-Type", "application/json");
  next();
 });


 initializeApp({
 credential: applicationDefault(),
 projectId: 'faekbank-app',
 }); 

 app.post("/send", function (req, res) {
  const receivedToken = req.body.fcmToken;

  const message = {
   notification: {
    title: "Notif",
    body: 'This is a Test Notification'
   },
    token: receivedToken,
   };

  getMessaging()
   .send(message)
  .then((response) => {
    res.status(200).json({
      message: "Successfully sent message",
      token: receivedToken,
    });
    console.log("Successfully sent message:", response);
  })
  .catch((error) => {
    res.status(400);
    res.send(error);
    console.log("Error sending message:", error);
  });


 });

 app.listen(3000, function () {
 console.log("Server started on port 3000");
  }); `

Can someone help.