navigation-preload.md

navigationPreload: boolean | Object | ':auto:'

Default: ':auto:'

Navigation preload is a ServiceWorker's feature which provides a way to make a request to the website even before ServiceWorker or a page is initialized. This can be useful for data fetching to speedup loading of the application.

Usage

In offline-plugin, Navigation preload behaves differently for cache-first and network-first response strategies.

For network-first navigation preload is enabled by default and allows to fetch navigation pages ahead of time, even before ServiceWorker was initialized. To disabled it set ServiceWorker.navigationPreload option to false.

For cache-first navigation preload has to be enabled manually and also handled on the server side. To enabled navigation preload two functions has to be specified:

ServiceWorker: {
  navigationPreload: {
    map: (url) => {
      if (url.pathname === '/') {
        return '/api/feed';
      }

      var post = url.pathname.match(/^\/post\/(\d+)$/);

      if (post) {
        return '/api/post/' + post[1];
      }
    },
    test: (url) => {
      if (url.pathname.indexOf('/api/') === 0) {
        return true;
      }
    }
  },
}

• map function is used to map navigation preload request to other requests, e.g. API requests.
• test function is used to test for possible consumers of navigation preload mapping

In previous example map function maps navigation preload of / to /api/feed. Then when a request to /api/feed happens, test function is used to determine that such request might be preloaded with navigation preload (it checks if pathname of the request starts with /api/).

Server Side

When a navigation preload request happens, it contains Service-Worker-Navigation-Preload: true header. Server side should use this header to detect the preload and send different content to such request.

Express.js example:

function serveIndex(req, res) {
  if (req.headers['service-worker-navigation-preload']) {
    res.set({
      'Cache-Control': 'no-cache',
      'Vary': 'Service-Worker-Navigation-Preload'
    });

    fetchFeedData(req).then((data) => {
      res.send(data);
    });

    return;
  }

  res.sendFile(path.join(WWW_FOLDER, 'index.html'), {
    cacheControl: false,
    acceptRanges: false,
    headers: {
      'Cache-Control': 'no-cache',
      'Vary': 'Service-Worker-Navigation-Preload'
    }
  });
}

Make sure to set 'Vary': 'Service-Worker-Navigation-Preload' header if you're planning on caching those responses.

For more details on Navigation Preload see this article.