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Home Interviews What are the three main phases in a service worker's lifecycle? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 06 Aug 2025
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The three main phases in a Service Worker's lifecycle are:

1. Registration

  • This is when your web page tells the browser that it wants to use a service worker.
  • Done using navigator.serviceWorker.register('/service-worker.js').

Code Example:

navigator.serviceWorker.register('/service-worker.js')
  .then(reg => console.log('Service Worker registered:', reg));

2. Installation

  • The browser downloads the service worker script and triggers the install event.
  • This is where you typically cache static assets (HTML, CSS, JS, etc.) needed for offline use.

Key Event:

self.addEventListener('install', event => {
  console.log('Service Worker installing...');
  // Cache files here
});

Note: If the install fails, the service worker is discarded.

3. Activation

  • After successful installation, the service worker enters the activate phase.
  • This is a good time to clean up old caches or migrate data from a previous version.

Key Event:

self.addEventListener('activate', event => {
  console.log('Service Worker activating...');
  // Delete old caches or update DB
});

Once activated:

  1. The service worker starts controlling pages (if eligible).
  2. The fetch event listener can now intercept network requests.

Bonus: Waiting Phase (Optional Middle State)

Between installation and activation, a service worker may enter a waiting state if an older version is still controlling pages. You can manually trigger it to take control using skipWaiting().

self.skipWaiting(); // Forces activation immediately

Summary Table

Phase Triggered When... Common Tasks
Register App requests registration Starts the lifecycle
Install New service worker is downloaded Cache essential assets
Activate Installation succeeds Clean up old cache, claim clients
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