service-worker.js

const CACHE_NAME = 'getid-cache-v1';
const URL_TO_CACHE = ['/getid'];

self.addEventListener('install', (event) => {
    event.waitUntil(
        caches.open(CACHE_NAME).then((cache) => {
            return cache.addAll(URL_TO_CACHE);
        })
    );
    self.skipWaiting();
});

self.addEventListener('fetch', (event) => {
    if (event.request.url.endsWith('/getid')) {
        // オンラインなら何もしない（ブラウザのfetchが動く）
        if (!self.navigator || !self.navigator.onLine) {
            event.respondWith(
                caches.match(event.request).then((response) => {
                    return response || new Response('[]', {
                        headers: { 'Content-Type': 'application/json' }
                    });
                })
            );
        }
    }
});