Service Worker Updates Skill

Purpose

Guide for modifying the service worker caching strategy. Prevent common mistakes that break offline functionality.

⚠️ CRITICAL: Never Edit public/sw.js Directly

The public/sw.js file is generated and will be overwritten.

Always edit: public/sw-template.js

How Service Worker Generation Works

public/sw-template.js  →  scripts/generate-sw.mjs  →  public/sw.js
       (source)              (build script)           (generated)

The build script:

1. Reads sw-template.js
2. Replaces {{API_ORIGIN}} placeholder with NEXT_PUBLIC_API_URL origin
3. Writes output to public/sw.js

When to Run Regeneration

Run yarn prebuild after changing sw-template.js:

# After editing sw-template.js
yarn prebuild

# Or just run dev (includes prebuild)
yarn dev

Note: yarn build alone does NOT run prebuild. Environment-specific builds (build:development|staging|production) do include it.

Service Worker Template Structure

// public/sw-template.js
const API_ORIGIN = "{{API_ORIGIN}}"; // Replaced at build time

// Workbox 7 configuration
importScripts(
  "https://storage.googleapis.com/workbox-cdn/releases/7.0.0/workbox-sw.js"
);

const { registerRoute } = workbox.routing;
const { CacheFirst, NetworkFirst, StaleWhileRevalidate } = workbox.strategies;

// Cache strategies for different routes
registerRoute(
  ({ url }) => url.origin === API_ORIGIN && url.pathname.startsWith("/api/"),
  new NetworkFirst({ cacheName: "api-cache" })
);

Caching Strategies

Strategy	Use Case	Example
CacheFirst	Static assets, images	Fonts, icons
NetworkFirst	API calls, dynamic data	Events, news
StaleWhileRevalidate	Balance freshness/speed	Category lists

Adding New Cache Rules

1. Edit Template

// public/sw-template.js

// Add new route
registerRoute(
  ({ url }) => url.pathname.startsWith("/new-feature/"),
  new StaleWhileRevalidate({
    cacheName: "new-feature-cache",
    matchOptions: { ignoreVary: true }, // REQUIRED for Next.js App Router
    plugins: [
      new workbox.expiration.ExpirationPlugin({
        maxEntries: 50,
        maxAgeSeconds: 24 * 60 * 60, // 24 hours
      }),
    ],
  })
);

2. Regenerate

yarn prebuild

3. Test

yarn dev
# Check DevTools > Application > Service Workers

Common Patterns

Cache API Responses

registerRoute(
  ({ url }) => url.origin === API_ORIGIN && url.pathname.includes("/events"),
  new NetworkFirst({
    cacheName: "events-cache",
    networkTimeoutSeconds: 3,
    matchOptions: { ignoreVary: true }, // REQUIRED for Next.js App Router
    plugins: [
      new workbox.expiration.ExpirationPlugin({
        maxEntries: 100,
        maxAgeSeconds: 60 * 60, // 1 hour
      }),
    ],
  })
);

Cache Static Assets

registerRoute(
  ({ request }) => request.destination === "image",
  new CacheFirst({
    cacheName: "images-cache",
    matchOptions: { ignoreVary: true }, // REQUIRED for Next.js App Router
    plugins: [
      new workbox.expiration.ExpirationPlugin({
        maxEntries: 60,
        maxAgeSeconds: 30 * 24 * 60 * 60, // 30 days
      }),
    ],
  })
);

Offline Fallback

// Offline page fallback
workbox.routing.setCatchHandler(async ({ event }) => {
  if (event.request.destination === "document") {
    return caches.match("/offline");
  }
  return Response.error();
});

Environment Variable

The {{API_ORIGIN}} placeholder is replaced with the origin from NEXT_PUBLIC_API_URL:

# .env
NEXT_PUBLIC_API_URL=https://api.example.com/v1

# Results in sw.js:
const API_ORIGIN = "https://api.example.com";

Debugging Service Worker

1. Chrome DevTools > Application > Service Workers
2. Check "Update on reload" during development
3. Use "Bypass for network" to test without caching
4. Clear storage if seeing stale behavior

⚠️ CRITICAL: Always Use ignoreVary: true

Next.js App Router adds Vary headers (rsc, next-router-state-tree, next-router-prefetch, etc.) to responses. Without ignoreVary: true, service worker cache matching fails because these headers differ between request types (RSC navigation vs client fetch vs direct load).

Note: This addresses Workbox service worker cache matching, not the underlying Next.js dynamic route issue (which relates to Cache-Control: private/no-store when Server Components call headers() or cookies()). The ignoreVary: true option allows the SW to treat the same URL as one cache entry regardless of Vary header differences.

Always add matchOptions with ignoreVary: true:

new workbox.strategies.StaleWhileRevalidate({
  cacheName: "my-cache",
  matchOptions: {
    ignoreVary: true, // REQUIRED for Next.js App Router compatibility
  },
  plugins: [
    /* ... */
  ],
});

Without this, the same URL may be fetched multiple times and cached as separate entries, causing:

• Wasted bandwidth
• Slower navigation (false cache misses)
• Unreliable offline behavior

Common Mistakes

1. Editing sw.js directly → Changes lost on rebuild
2. Forgetting yarn prebuild → Template changes not applied
3. Wrong API_ORIGIN → Cache rules don't match
4. Too aggressive caching → Users see stale data
5. No cache expiration → Cache grows forever
6. Missing ignoreVary: true → Cache misses due to Next.js Vary headers

Checklist for SW Changes

• Editing sw-template.js (not sw.js)?
• Running yarn prebuild after changes?
• Using appropriate cache strategy?
• Setting cache expiration limits?
• Adding matchOptions: { ignoreVary: true } to strategies?
• Testing in DevTools > Application?
• Testing offline behavior?

Files to Reference

• public/sw-template.js - Source template (EDIT THIS)
• public/sw.js - Generated output (DO NOT EDIT)
• scripts/generate-sw.mjs - Build script
• app/offline/page.tsx - Offline fallback page