index.js

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

exports.fcmSend = functions.database
  .ref('/messages/{userId}/{messageId}')
  .onCreate(event => {
    const message = event.after.val();
    const userId = event.params.userId;

    const payload = {
      notification: {
        title: message.title,
        body: message.body,
        icon: 'https://placeimg.com/250/250/people'
      }
    };

    admin
      .database()
      .ref(`/fcmTokens/${userId}`)
      .once('value')
      .then(token => token.val())
      .then(userFcmToken => {
        return admin.messaging().sendToDevice(userFcmToken, payload);
      })
      .then(res => {
        console.log('Sent Successfully', res);
      })
      .catch(err => {
        console.log(err);
      });
  });