title: Firebase functions Cannot read property 'val' of undefined
body: <p>I'm new at functions in Firebase, had read some changes in the official documentation, but as I'm doing a notification system it throw to me this error when I try to get my token id I think.</p>
  
  <p>My code  </p>
  
  <pre><code>     'use strict'
  
  const functions = require('firebase-functions');
  const admin = require('firebase-admin');
  admin.initializeApp(functions.config().firebase);
  
  
  /*
   * 'OnWrite' works as 'addValueEventListener' for android. It will fire the function
   * everytime there is some item added, removed or changed from the provided 'database.ref'
   * 'sendNotification' is the name of the function, which can be changed according to
   * your requirement
   */
  
  exports.sendNotification = functions.database.ref('/notifications/{user_id}/{notification_id}').onWrite((change, context) =&gt; {
  
  
    /*
     * You can store values as variables from the 'database.ref'
     * Just like here, I've done for 'user_id' and 'notification'
     */
  
    const user_id = context.params.user_id;
    const notification_id = context.params.notification_id;
  
    console.log('Tenemos una notificacion para mandar a : ', context.params.user_id);
  
     if(!context.data.val()){
  
      return console.log('Una notificacion se elimino de la base de datos : ', notification_id);
  
    }
  
    const deviceToken = admin.database().ref(`/users/${user_id}/device_token`).once('value');
  
    return deviceToken.then(result =&gt; {
  
      const token_id = result.val();
  
        /*
         * We are creating a 'payload' to create a notification to be sent.
         */
  
      const payload = {
          notification: {
            title : "Tienes un nuevo seguidor !",
            body: "Tu nuevo seguidor es .... ",
            icon: "default"
          }
  
        };
  
         /*
         * Then using admin.messaging() we are sending the payload notification to the token_id of
         * the device we retreived.
         */
  
        return admin.messaging().sendToDevice(token_id, payload).then(response =&gt; {
  
          return console.log('This was the notification Feature');
  
        });
  
    });
  
  
  
    });
  </code></pre>
  
  <p>The error </p>
  
  <blockquote>
    <p>Cannot read property 'val' of undefined
        at exports.sendNotification.functions.database.ref.onWrite (/user_code/index.js:28:20)
        at Object. (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:112:27)
        at next (native)
        at /user_code/node_modules/firebase-functions/lib/cloud-functions.js:28:71
        at __awaiter (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:24:12)
        at cloudFunction (/user_code/node_modules/firebase-functions/lib/cloud-functions.js:82:36)
        at /var/tmp/worker/worker.js:716:24
        at process._tickDomainCallback (internal/process/next_tick.js:135:7)</p>
  </blockquote>