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name: service-worker description: "Use when auditing repeat-visit performance, adding offline support, or evaluating PWA readiness for a web application." metadata: category: performance priority: medium difficulty: intermediate estimatedTime: "60" source: frontendchecklist.io url: https://frontendchecklist.io/rules/performance/service-worker

Register a service worker for caching and offline support

Service workers act as a programmable network proxy between the browser and the network. They eliminate repeated server round-trips for static assets, cut load times on repeat visits by 50–90 %, and allow the app to function at all on flaky or offline networks — which is critical for users on mobile or low-bandwidth connections.

Quick Reference

• Register a service worker in your main JavaScript entry point
• Use an install event to pre-cache critical static assets
• Choose a caching strategy (cache-first, network-first, stale-while-revalidate) per resource type
• Implement an activate event to clean up old caches on update

Check

Check whether a service worker is registered and whether it caches static assets, API responses, or provides an offline fallback.

Fix

Add a service worker registration call and implement appropriate caching strategies for static assets and navigation requests.

Explain

Explain how service workers intercept network requests and how different caching strategies trade off freshness vs. speed.

Code Review

Review the service worker file and its registration. Flag missing install/ activate lifecycle handlers, absent cache-versioning, missing fetch handlers, and any patterns that could cause stale content to be served indefinitely.

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For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/rules/performance/service-worker