service-worker

Original：🇺🇸 English
Translated

Service Worker API implementation guide — registration, lifecycle management, caching strategies, push notifications, and background sync. Use when: (1) creating or modifying service worker files (sw.js), (2) implementing offline-first caching (cache-first, network-first, stale-while-revalidate), (3) setting up push notifications or background sync, (4) debugging service worker registration, scope, or update issues, (5) implementing navigation preload, (6) user mentions 'service worker', 'sw.js', 'offline support', 'cache strategy', 'push notification', 'background sync', 'workbox alternative', or 'PWA caching'.

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Sourcejgamaraalv/ts-dev-kit
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NPX Install

npx skill4agent add jgamaraalv/ts-dev-kit service-worker

Tags

Translated version includes tags in frontmatter
service-worker-apicaching-strategiespush-notificationspwa-developmentoffline-support

SKILL.md Content

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Service Worker

Table of Contents

Constraints

  • HTTPS required (localhost exempt for dev)
  • No DOM access — runs on separate thread
  • Fully async — no synchronous XHR, no localStorage
  • No dynamic 
    import()
    — only static 
    import
    statements
  • Scope defaults to the directory containing the SW file
  • self
    refers to 
    ServiceWorkerGlobalScope

Lifecycle

register() → Download → Install → [Wait] → Activate → Fetch control
  1. Register from main thread via 
    navigator.serviceWorker.register()
  2. Install event fires once — use to pre-cache static assets
  3. Wait — new SW waits until all tabs using old SW are closed (skip with 
    self.skipWaiting()
    )
  4. Activate event fires — use to clean up old caches
  5. Fetch events start flowing — SW controls page network requests
A document must reload to be controlled (or call 
clients.claim()
during activate).

Registration

js
// main.js — register from the page
if ("serviceWorker" in navigator) {
  const reg = await navigator.serviceWorker.register("/sw.js", { scope: "/" });
  // reg.installing | reg.waiting | reg.active
}
Scope rules:
  • SW at 
    /sw.js
    can control 
    /
    and all subpaths
  • SW at 
    /app/sw.js
    can only control 
    /app/
    by default
  • Broaden scope with 
    Service-Worker-Allowed
    response header

Install Event — Pre-cache Assets

js
// sw.js
const CACHE_NAME = "v1";
const PRECACHE_URLS = ["/", "/index.html", "/style.css", "/app.js"];

self.addEventListener("install", (event) => {
  event.waitUntil(caches.open(CACHE_NAME).then((cache) => cache.addAll(PRECACHE_URLS)));
});
waitUntil(promise)
— keeps install phase alive until the promise settles. If rejected, installation fails and the SW won't activate.

Activate Event — Clean Up Old Caches

js
self.addEventListener("activate", (event) => {
  event.waitUntil(
    caches
      .keys()
      .then((keys) =>
        Promise.all(keys.filter((key) => key !== CACHE_NAME).map((key) => caches.delete(key))),
      ),
  );
});

Fetch Event — Intercept Requests

js
self.addEventListener("fetch", (event) => {
  event.respondWith(caches.match(event.request).then((cached) => cached || fetch(event.request)));
});
respondWith(promise)
— must be called synchronously (within the event handler, not in a microtask). The promise resolves to a 
Response
.
For caching strategy patterns (cache-first, network-first, stale-while-revalidate), see references/caching-strategies.md.

Navigation Preload

Avoid the startup delay when a SW boots to handle a navigation:
js
self.addEventListener("activate", (event) => {
  event.waitUntil(self.registration?.navigationPreload.enable());
});

self.addEventListener("fetch", (event) => {
  event.respondWith(
    (async () => {
      const cached = await caches.match(event.request);
      if (cached) return cached;

      const preloaded = await event.preloadResponse;
      if (preloaded) return preloaded;

      return fetch(event.request);
    })(),
  );
});

Updating a Service Worker

  • Browser byte-compares the SW file on each navigation (or every 24h)
  • New version installs in background while old version still serves
  • Increment the cache name (e.g., 
    v1
    → 
    v2
    ) in the new version
  • Delete old caches in the 
    activate
    handler
  • Call 
    self.skipWaiting()
    in 
    install
    to activate immediately
  • Call 
    self.clients.claim()
    in 
    activate
    to take control of open pages

Communicating with Pages

js
// Page → SW
navigator.serviceWorker.controller.postMessage({ type: "SKIP_WAITING" });

// SW → Page (via Clients API)
const clients = await self.clients.matchAll({ type: "window" });
clients.forEach((client) => client.postMessage({ type: "UPDATED" }));

// SW listens
self.addEventListener("message", (event) => {
  if (event.data?.type === "SKIP_WAITING") self.skipWaiting();
});

Common Pitfalls

  1. Response cloning — 
    response.clone()
    before both caching and returning, since body streams can only be read once
  2. Opaque responses — cross-origin fetches without CORS return opaque responses (status 0). 
    cache.add()
    will refuse them. Use 
    cache.put()
    but you can't inspect the response
  3. waitUntil timing — call 
    event.waitUntil()
    synchronously within the event handler, not inside an async callback
  4. Scope ceiling — a SW cannot control URLs above its own directory unless 
    Service-Worker-Allowed
    header is set
  5. No state persistence — the SW may terminate at any time when idle. Don't store state in global variables — use Cache API or IndexedDB

Push Notifications & Background Sync

For push subscription, handling push events, and background sync implementation, see references/push-and-sync.md.

API Quick Reference

For detailed interfaces (
Cache
, 
CacheStorage
, 
FetchEvent
, 
Clients
, 
ServiceWorkerRegistration
, 
ServiceWorkerGlobalScope
), see references/api-reference.md.

Next.js Integration

In Next.js, place the service worker file in 
public/sw.js
. 
public/sw.js
is intentionally plain JS (not processed by Next.js build pipeline). Register it from a client component:
tsx
"use client";
import { useEffect } from "react";

export function ServiceWorkerRegistrar() {
  useEffect(() => {
    if ("serviceWorker" in navigator) {
      navigator.serviceWorker.register("/sw.js");
    }
  }, []);
  return null;
}
Add to root layout. Next.js serves 
public/
files at the root, so 
/sw.js
scope covers 
/
.

DevTools

  • Chrome: 
    chrome://inspect/#service-workers
    or Application > Service Workers
  • Firefox: 
    about:debugging#/runtime/this-firefox
    or Application > Service Workers
  • Edge: 
    edge://inspect/#service-workers
    or Application > Service Workers
Unregister, update, and inspect caches from the Application panel. Use "Update on reload" checkbox during development.