On the Firebase console home page (shown previously), click on Add Firebase to your web app.

You should see a popup that will give you the scripts that you then have to include in the index.html file:

Add these scripts to the index.html file just before the script tag that includes the script.js file:

    ...
    ...
      </div>

      <script     src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.6.
    0/Chart.bundle.js">

      </script>
      <script     src="https://www.gstatic.com/firebasejs/4.1.3/firebase.js
    "></script>
      <script>
        // Initialize Firebase
        var config = {
          apiKey: "......",
          authDomain: "sensor-project-5df04.firebaseapp.com",
          databaseURL: "https://sensor-project-
    5df04.firebaseio.com",
          projectId: "sensor-project-5df04",
          storageBucket: "sensor-project-5df04.appspot.com",
          messagingSenderId: "......"
        };
        firebase.initializeApp(config);

      </script>
      <script src="script.js"></script>
    </body>
    ...
    ...