title: How to send push notifications to multiple devices using Firebase Cloud Messaging
body: <p>I was findind a way to deliver push messages from my expressJS server to my ionic app and I found <strong>GCM</strong>. With GCM I could deliver the message passing a list of tokens, like this : </p>
  
  <pre><code> sender.send(message, {
          registrationTokens: deviceTokens
      }, function (err, response) {
          if (err) console.error(err);
          else console.log('response' + JSON.stringify(response));
      });
  </code></pre>
  
  <p>But as I found that <strong>GCM</strong> became <strong>FCM</strong> I was trying to do the same using FCM, but no luck until now. I've heard about sending topics but I couldn't find an example.</p>
  
  <p>Can anyone give an example on how send topic messages using FCM ?</p>
  
  <p>my FCM code: (working with just 1 token)</p>
  
  <pre><code> var FCM = require('fcm-node');
  
  var serverKey = 'xxx';
  var fcm = new FCM(serverKey);
  
  var message = {
  
      to: 'device-token',
  
      notification: {
          title: event.title,
          body: event.information
      }
  
  };
  
  fcm.send(message, function (err, response) {
      if (err) {
          console.log("Something has gone wrong! \n" + err);
      } else {
          console.log("Successfully sent with response: \n ", JSON.stringify(response));
      }
  });
  </code></pre>