index.js

if ('serviceWorker' in navigator) {
    navigator.serviceWorker.register('/sw.js', {
        // The Service-Worker-Allowed must be set to '/'
        scope: '/',
        // Allow the service worker to use modules
        type: 'module',
        // Don't cache http requests.
        updateViaCache: 'none'
    }).then(reg => {
        // Update service worker
        reg.update();

        // When the service worker is ready.
        if ('active' in reg) {
            // Share server data with the service worker.
            const chan = new MessageChannel();
            reg.active.postMessage(ctx, [chan.port2]);
            
            // Reload page
            location.reload();
        } else
            console.log(reg.state);
    });
} else {
    const firefox = false;
    
    // Service workers can't be created if on private browsing mode on firefox   
    if (firefox) {
        document.write('❌ Please leave private browsing mode.');
    } else {
        document.write('❌ Service workers are not supported!');
    }
}