Defer installation script for PWAs (copy & pasted from https://developers.google.com/web/fundamentals/app-install-banners/ )

index.html

<script type="text/javascript">
      // ...
      
      // Ref. https://developers.google.com/web/fundamentals/app-install-banners/

      function addToHomeScreen() {
        let a2hsBtn = document.querySelector(".ad2hs-prompt");  // hide our user interface that shows our A2HS button
        a2hsBtn.style.display = 'none';  // Show the prompt
        deferredPrompt.prompt();  // Wait for the user to respond to the prompt
        deferredPrompt.userChoice
          .then(function(choiceResult){

        if (choiceResult.outcome === 'accepted') {
          console.log('User accepted the A2HS prompt');
        } else {
          console.log('User dismissed the A2HS prompt');
        }

        deferredPrompt = null;

      });}

      function showAddToHomeScreen() {

        let a2hsBtn = document.querySelector(".ad2hs-prompt");
        a2hsBtn.style.display = "block";
        a2hsBtn.addEventListener("click", addToHomeScreen);

      }

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

        showAddToHomeScreen();
      });

    </script>