a2hs.js

var installPromptEvent;
var btnInstall = document.querySelector('#install');

window.addEventListener('beforeinstallprompt', function (event) {
    event.preventDefault();
    installPromptEvent = event;
    btnInstall.removeAttribute('disabled');
});

btnInstall.addEventListener('click', function () {
    btnInstall.setAttribute('disabled', '');
    installPromptEvent.prompt();
    installPromptEvent.userChoice.then((choice) => {
        if (choice.outcome === 'accepted') {
            console.log('User accepted the A2HS prompt');
        } else {
            console.log('User dismissed the A2HS prompt');
        }
        installPromptEvent = null;
    });
});