A service worker file

sw.js

// Listens to an install event
self.addEventListener("install", (event) => {
    event.waitUntil(
        // We try to open a cache named "assets". 
        // If it does not exist, a new one will be created and named "assets"
        caches.open("assets").then((cache) => {
            // Add an array of path to files you want to add to cache
            return cache.addAll([
                "./",
                "./css/index.css",
                "./assets/logo192.png",
                "./assets/logo512.png",
            ]);
        })
    );
});