Firebase Firestore Cloud Messaging(Notification) Server Function

index.js

const functions = require("firebase-functions");
var request = require("request");

var API_KEY = "YOUR-API-KEY"; // Your Firebase Cloud Messaging Server API key

function sendNotificationToUser(userId, message) {
  request(
    {
      url: "https://fcm.googleapis.com/fcm/send",
      method: "POST",
      headers: {
        "Content-Type": " application/json",
        Authorization: "key=" + API_KEY
      },
      body: JSON.stringify({
        notification: {
          title: message
        },
        to: "/topics/user-" + userId
      })
    },
    function(error, response, body) {
      if (error) {
        console.error(error);
      } else if (response.statusCode >= 400) {
        console.error(
          "HTTP Error: " + response.statusCode + " - " + response.statusMessage
        );
      }
    }
  );
}

exports.notificationCreate = functions.firestore
  .document("/notifications/{id}")
  .onCreate((snapshot, context) => {
    const data = snapshot.data();
    sendNotificationToUser(data.userId, data.message);
  });