To build a web application that runs offline, you should use Service Worker technology to achieve offline caching. 1. Register the Service Worker and listen to the install event to cache necessary resources; 2. Listen to the fetch event to decide to return data from the network or cache; 3. Select the appropriate cache policy according to the resource type, such as "cache priority" for static resources, and "network priority" for dynamic content; 4. Pay attention to HTTPS requirements, cache version updates and debugging tools to ensure that Service Worker takes effect correctly. The entire process needs to be finely configured to avoid functional failure.

The key to building a web application that runs offline is to take advantage of the "offline caching" function provided by HTML5. Through AppCache or Service Worker technology, web pages can still be loaded and run without a network connection. However, AppCache has been gradually deprecated by modern browsers, and the mainstream approach now is to use a more flexible and more controllable Service Worker.

The following aspects are what you need to focus on.

Manage offline resources using Service Worker

Service Worker is a script running in the browser's background. It acts as a network proxy that intercepts network requests and decides whether to get resources from the network or read from the local cache.

To achieve offline access, you need:

• Register a Service Worker file (usually named service-worker.js )
• Listen to the install event in this file and cache the necessary resources to the client
• Listen to fetch events, try to return data from cache when requesting resources

The sample code is as follows:

 // index.js
if ('serviceWorker' in navigator) {
  window.addEventListener('load', () => {
    navigator.serviceWorker.register('/service-worker.js')
      .then(registration => console.log('SW registered:', registration.scope))
      .catch(error => console.log('SW registration failed:', error));
  });
}

 // service-worker.js
const CACHE_NAME = 'offline-cache-v1';
const OFFLINE_RESOURCES = ['/', '/index.html', '/styles.css', '/app.js'];

self.addEventListener('install', event => {
  event.waitUntil(
    caches.open(CACHE_NAME)
      .then(cache => cache.addAll(OFFLINE_RESOURCES))
  );
});

self.addEventListener('fetch', event => {
  event.respondWith(
    fetch(event.request)
      .catch(() => caches.match(event.request).then(response => response))
  );
});

Cache Policy Selection: Static Resources vs Dynamic Content

Not everything is suitable for caching. Generally speaking, static resources such as HTML, CSS, JS, and images can be cached for a longer period of time, while API interfaces or user-generated content need to be handled more carefully.

You can adopt the following strategies:

• Cache priority : try to read from the cache first, and then initiate a network request if it fails.
• Network priority : try the network request first, and then read from the cache after failure.
• Cache and update : while returning the old version from the cache, update the cache in the background

Different pages or resources can choose different policies, such as "Cache First" on the homepage and "Network First" on the user notification list.

Notes and FAQs

When implementing offline web applications, there are several error-prone areas that need to be paid attention to:

• HTTPS Requirements : Service Worker can only run under HTTPS (except localhost), otherwise registration will fail
• Cache update mechanism : Service Worker needs to manually update the cache version, otherwise new resources will not be automatically replaced.
• Debug Tools : Chrome DevTools' Application tab provides the viewing methods of Service Worker and Cache Storage for easy troubleshooting

If you find that the app still displays old content after update, it may be because the cache is not updated correctly or the SW is not activated.

Basically that's it. Although the whole process is not complicated, each link needs to be carefully configured, especially the life cycle management of cache policies and service worker threads, which can easily ignore details and cause functional failure.

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