workbox-navigation-preload

"Speed up Service Worker with Navigation Preloads" does a great job of explaining what navigation preload is, and the benefits it offers to web apps whose service worker does not explicitly handle navigation requests.

# What does this module do?

workbox-navigation-preload will handle checking at runtime to see if the current browser supports navigation preload, and if it does, it will automatically create an activate event handler to enable it.

The shared code inside of workbox-core that handles making network requests across all of Workbox has also been updated to automatically take advantage of a preload response, if it's available. This means that any of the built-in strategies can automatically take advantage of navigation preload, once it's enabled.

# Who should enable navigation preloads?

Developers who are already handling navigations by responding with precached HTML (potentially configured with an App Shell fallback) do not need to enable navigation preload! This feature is intended to reduce navigation latency for developers who can't precache their HTML, but still want to use Workbox to handle caching of other assets on their sites.

For instance, if you're following the App Shell pattern, and you've got a navigation route already set up to use the precached HTML, enabling navigation preload will be a waste. The network response that is associated with the preload request will never end up being used, since the precached HTML will be used unconditionally.

# Basic Usage

import * as navigationPreload from 'workbox-navigation-preload';
import {NetworkFirst} from 'workbox-strategies';
import {registerRoute, NavigationRoute} from 'workbox-routing';

// Enable navigation preload.
navigationPreload.enable();

// Swap in NetworkOnly, CacheFirst, or StaleWhileRevalidate as needed.
const strategy = new NetworkFirst({
  cacheName: 'cached-navigations',
  plugins: [
    // Any plugins, like `ExpirationPlugin`, etc.
  ],
});

const navigationRoute = new NavigationRoute(strategy, {
  // Optionally, provide a allow/denylist of RegExps to determine
  // which paths will match this route.
  // allowlist: [],
  // denylist: [],
});

registerRoute(navigationRoute);

# What's the browser support story?

Currently, Google Chrome is the only browser that supports navigation preload. enable() will check for browser support at runtime, and only attempt to enable navigation preload if the current browser supports it. It's therefore safe to call enable() unconditionally in your service worker.

You should be aware that those browsers will not benefit from the navigation latency reduction, and it's recommended that you carefully measure the performance implications of shipping a service worker that doesn't handle navigation requests, and doesn't use navigation preload.