Cache priority and fallback options in the JavaScript Cache API

23 Oct 2023

When building web applications, optimizing the performance and providing a seamless experience for users is crucial. Caching is an effective technique that can drastically improve the speed and responsiveness of your web pages.

In JavaScript, the Cache API provides a way to store assets such as JavaScript, CSS, and image files locally, allowing them to be retrieved quickly without making network requests. One of the powerful features of the Cache API is the ability to set cache priorities and fallback options.

Setting Cache Priority

The cache priority determines the order in which the browser should try to fetch and serve resources from the cache. By specifying the cache priority, you can control which resources should be loaded first and which can be fetched later. This is particularly useful when dealing with limited network resources or critical assets that should be prioritized for a better user experience.

To set the cache priority for a specific resource, you can use the Request object and its cache property. The cache property accepts one of three possible values:

default: This is the default priority. It indicates that the resource can be fetched and served from the cache, but it won’t be prioritized over other ongoing network requests.
reload: This option tells the browser to bypass the cache and always fetch the resource from the network, ignoring all cache entries.
force-cache: With this option, the browser will serve the resource from the cache if it’s available, without making a network request. If the resource is not in the cache, the browser will fetch it from the network and store it in the cache for future use.

Here’s an example of setting the cache priority for a fetch request:

const request = new Request('/api/data.json', {
  cache: 'reload'
});

fetch(request)
  .then(response => {
    // Handle the response
  })
  .catch(error => {
    // Handle errors
  });

Fallback Options

Fallback options are useful when you have a resource that may not be available in the cache or through the network. Instead of showing an error page or leaving the user waiting indefinitely, you can define fallback resources to be used in case the requested resource is not available.

To implement fallback options, you can use the Cache and Response objects. First, you need to check if the requested resource is available in the cache. If it’s not, you can fetch the fallback resource and serve it instead.

Here’s an example:

self.addEventListener('fetch', (event) => {
  event.respondWith(
    caches.match(event.request)
      .then(response => {
        if (response) {
          return response; // Serve from cache
        } else {
          return fetch('/fallback.html'); // Fetch fallback resource
        }
      })
      .catch(error => {
        return new Response('An error occurred. Please try again later.');
      })
  );
});

In the above example, if the requested resource is in the cache, it is immediately served. Otherwise, the fallback resource fallback.html is fetched and returned as the response.

By using cache priorities and fallback options in the Cache API, you can optimize the performance of your web applications and ensure a smooth user experience, even in challenging network conditions.

Conclusion

The Cache API in JavaScript provides a powerful toolset to improve the performance and responsiveness of web applications. By setting cache priorities and defining fallback options, you can optimize resource loading and handle scenarios when requested resources are not available. Understanding and utilizing these features can greatly enhance the user experience of your web applications.

References:

#webdevelopment #caching