PWA – beforeinstallprompt not called

Try this :

 <script>
    let deferredPrompt;

    window.addEventListener('beforeinstallprompt', function(event) {
      // Prevent Chrome 67 and earlier from automatically showing the prompt
      e.preventDefault();
      // Stash the event so it can be triggered later.
      deferredPrompt = e;
    });

    // Installation must be done by a user gesture! Here, the button click
    btnAdd.addEventListener('click', (e) => {
      // hide our user interface that shows our A2HS button
      btnAdd.style.display = 'none';
      // Show the prompt
      deferredPrompt.prompt();
      // Wait for the user to respond to the prompt
      deferredPrompt.userChoice
        .then((choiceResult) => {
          if (choiceResult.outcome === 'accepted') {
            console.log('User accepted the A2HS prompt');
          } else {
            console.log('User dismissed the A2HS prompt');
          }
          deferredPrompt = null;
        });
    });

    </script>

beforeinstallprompt will only be fired when some conditions are true :

• The PWA must not already be installed
• Meets a user engagement heuristic (previously, the user had to interact with the domain for at least 30 seconds, this is not a requirement anymore).
• Your web app must include a web app manifest.
• Your web app must be served over a secure HTTPS connection.
• Has registered a service worker with a fetch event handler.