title: ExpressJS and Firebase Auth, beginner queries
body: <p>I am trying to learn ExpressJS, so I creating a simple website with login functionality...
  I want to use Firebase for the accounts and database(Firestore)</p>
  
  <p>My problem is that, Firebase Auth seems to be client-side only and the backend has no idea if the client is logged in or not...
  I want to limit the HTML rendered to the client if he is not logged in, but I can't figure out how to check if he is logged in</p>
  
  <p>I know that I can use </p>
  
  <pre><code>firebase.auth().onAuthStateChanged(function(user){
    if (!user){
      window.location.replace("/login");
  }});
  </code></pre>
  
  <p>on the client side, that doesn't look "Secured" enough to me and I would like to do it in ExpressJS</p>
  
  <p>On firebase documentation I found this about <a href="https://firebase.google.com/docs/auth/admin/verify-id-tokens#retrieve_id_tokens_on_clients" rel="nofollow noreferrer">Verifying ID Tokens</a>
  but I don't understand first of all how would I send the token to ExpressJS and second, how would I send it before the request to a route is made...</p>