Window.onbeforeinstallprompt

The Window.onbeforeinstallprompt property is an event handler for processing a beforeinstallprompt, which is dispatched on devices when a user is about to be prompted to "install" a web application. Its associated event may be saved for later and used to prompt the user at a more suitable time. 

window.addEventListener("beforeinstallprompt", function(event) { ... });
window.onbeforeinstallprompt = function(event) { ...};

The following example uses the beforeinstallprompt event to make an install button operable, by using the event inside a click handler.

window.addEventListener("beforeinstallprompt", function(beforeInstallPromptEvent) {
  beforeInstallPromptEvent.preventDefault(); // Prevents immediate prompt display

  // Shows prompt after a user clicks an "install" button
  installButton.addEventListener("click", function(mouseEvent) {
    // you should not use the MouseEvent here, obviously
    beforeInstallPromptEvent.prompt();
  });

  installButton.hidden = false; // Make button operable
});

• BeforeInstallPromptEvent.prompt
• BeforeInstallPromptEvent