# Service Worker basics — Progressive Web Apps

Source: https://www.geekswithgeeks.com/en/pwa/pwa-service-workers

> Understand what service workers are and how to register them for offline functionality.

## The essential idea

A **service worker** is a JavaScript file that runs in the background, separate from the main thread. It intercepts network requests, caches responses, and enables offline functionality—the core technology powering PWAs.

## Register a service worker

This code registers a service worker file and handles registration success or failure.

```javascript
if ('serviceWorker' in navigator) {
  navigator.serviceWorker.register('/sw.js')
    .then(reg => {
      console.log('Service Worker registered:', reg);
    })
    .catch(err => {
      console.error('Service Worker registration failed:', err);
    });
}
```

Output:

```
Service Worker successfully registered and ready to intercept network requests.
```

## Service Worker lifecycle

Service workers go through stages: parse, install, activate, and fetch events.

![Service Worker lifecycle diagram showing registration, installation, activation, and message passing.](assets/figures/pwa/sw-lifecycle.svg) — Figure 2.1 — Service Worker lifecycle and states.

## A common trap

Service workers must be served over HTTPS (except localhost). This is a security requirement you cannot bypass.

## Service worker registration patterns

Compare different registration strategies and error handling approaches.