Send push notifications using Cloud Functions for Firebase

I managed to make this work. Here is the code that sends a notification using Cloud Functions that worked for me.

exports.sendNewTripNotification = functions.database.ref('/{uid}/shared_trips/').onWrite(event=>{
    const uuid = event.params.uid;

    console.log('User to send notification', uuid);

    var ref = admin.database().ref(`Users/${uuid}/token`);
    return ref.once("value", function(snapshot){
         const payload = {
              notification: {
                  title: 'You have been invited to a trip.',
                  body: 'Tap here to check it out!'
              }
         };

         admin.messaging().sendToDevice(snapshot.val(), payload)

    }, function (errorObject) {
        console.log("The read failed: " + errorObject.code);
    });
})

Just answering the question from Jerin A Mathews... Send message using Topics:

const functions = require('firebase-functions');
const admin = require('firebase-admin');

admin.initializeApp(functions.config().firebase);

//Now we're going to create a function that listens to when a 'Notifications' node changes and send a notificcation
//to all devices subscribed to a topic

exports.sendNotification = functions.database.ref("Notifications/{uid}")
.onWrite(event => {
    //This will be the notification model that we push to firebase
    var request = event.data.val();

    var payload = {
        data:{
          username: request.username,
          imageUrl: request.imageUrl,
          email: request.email,
          uid: request.uid,
          text: request.text
        }
    };

    //The topic variable can be anything from a username, to a uid
    //I find this approach much better than using the refresh token
    //as you can subscribe to someone's phone number, username, or some other unique identifier
    //to communicate between

    //Now let's move onto the code, but before that, let's push this to firebase

    admin.messaging().sendToTopic(request.topic, payload)
    .then((response) => {
        console.log("Successfully sent message: ", response);
        return true;
    })
    .catch((error) => {
        console.log("Error sending message: ", error);
        return false;
    })
})
//And this is it for building notifications to multiple devices from or to one.

Return this function call.

return ref.on("value", function(snapshot){
        console.log("Val = " + snapshot.val());
        },
    function (errorObject) {
        console.log("The read failed: " + errorObject.code);
});

This will keep the cloud function alive until the request is complete. Learn more about returning promises form the link give by Doug in the comment.

Send Notification for a Topic In Cloud function

Topics a basically groups you can send notification for the selected group

    var topic = 'NOTIFICATION_TOPIC';
    const payload = {
        notification: {
            title: 'Send through Topic',
            body: 'Tap here to check it out!'
        }
   };

   admin.messaging().sendToTopic(topic,payload);

You can register the device for any new or existing topic from mobile side

const functions = require('firebase-functions');
const admin = require('firebase-admin');
admin.initializeApp(functions.config().firebase);


exports.sendNotificationToTopic = 
functions.firestore.document('Users/{uuid}').onWrite(async (event) => {

//let title = event.after.get('item_name');
//let content = event.after.get('cust_name');
var message = {
    notification: {
        title: "TGC - New Order Recieved",
        body: "A New Order Recieved on TGC App",
    },
    topic: 'orders_comming',
};

let response = await admin.messaging().send(message);
console.log(response);
});

For sending notifications to a topic, The above code works well for me, if you have any doubt, let me know.