The Cache API: A quick guide

Learn how to use the Cache API to make your application data available offline.

Pete LePage

The Cache API is a system for storing and retrieving network requests and their corresponding responses. These might be regular requests and responses created in the course of running your application, or they could be created solely for the purpose of storing data for later use.

The Cache API was created to enable service workers to cache network requests so that they can provide fast responses, regardless of network speed or availability. However, the API can also be used as a general storage mechanism.

Where is it available?

The Cache API is available in all modern browsers. It is exposed via the global caches property, so you can test for the presence of the API with a simple feature detection:

const cacheAvailable = 'caches' in self;

The Cache API can be accessed from a window, iframe, worker, or service worker.

What can be stored

The caches only store pairs of Request and Response objects, representing HTTP requests and responses, respectively. However, the requests and responses can contain any kind of data that can be transferred over HTTP.

How much can be stored?

In short, a lot, at least a couple of hundred megabytes, and potentially hundreds of gigabytes or more. Browser implementations vary, but the amount of storage available is usually based on the amount of storage available on the device.

Creating and opening a cache

To open a cache, use the caches.open(name) method, passing the name of the cache as the single parameter. If the named cache does not exist, it is created. This method returns a Promise that resolves with the Cache object.

const cache = await caches.open('my-cache');
// do something with cache...

Adding to a cache

There are three ways to add an item to a cache - add, addAll, and put. All three methods return a Promise.

cache.add

First, there is cache.add(). It takes one parameter, either a Request or a URL (string). It makes a request to the network and stores the response in the cache. If the fetch fails, or if the status code of the response is not in the 200 range, then nothing is stored and the Promise rejects. Note that cross-origin requests not in CORS mode cannot be stored because they return a status of 0. Such requests can only be stored with put.

// Retreive data.json from the server and store the response.
cache.add(new Request('/data.json'));

// Retreive data.json from the server and store the response.
cache.add('/data.json');

cache.addAll

Next, there is cache.addAll(). It works similarly to add(), but takes an array of Request objects or URLs (strings). This works similarly to calling cache.add for each individual request, except that the Promise rejects if any single request is not cached.

const urls = ['/weather/today.json', '/weather/tomorrow.json'];
cache.addAll(urls);

In each of these cases, a new entry overwrites any matching existing entry. This uses the same matching rules described in the section on retrieving.

cache.put

Finally, there is cache.put(), which allows you to store either a response from the network, or create and store your own Response. It takes two parameters. The first can either be a Request object or a URL (string). The second must be a Response, either from the network, or generated by your code.

// Retrieve data.json from the server and store the response.
cache.put('/data.json');

// Create a new entry for test.json and store the newly created response.
cache.put('/test.json', new Response('{"foo": "bar"}'));

// Retrieve data.json from the 3rd party site and store the response.
cache.put('https://example.com/data.json');

The put() method is more permissive than either add() or addAll(), and will allow you to store non-CORS responses, or other responses where the status code of the response is not in the 200 range. It will overwrite any previous responses for the same request.

Creating Request objects

Create the Request object using a URL for the thing being stored:

const request = new Request('/my-data-store/item-id');

Working with Response objects

The Response object constructor accepts many types of data, including Blobs, ArrayBuffers, FormData objects, and strings.

const imageBlob = new Blob([data], {type: 'image/jpeg'});
const imageResponse = new Response(imageBlob);
const stringResponse = new Response('Hello world');

You can set the MIME type of a Response by setting the appropriate header.

  const options = {
    headers: {
      'Content-Type': 'application/json'
    }
  }
  const jsonResponse = new Response('{}', options);

If you have retrieved a Response and wish to access its body, there are several helper methods you can use. Each returns a Promise that resolves with a value of a different type.

For example

const response = new Response('Hello world');
const buffer = await response.arrayBuffer();
console.log(new Uint8Array(buffer));
// Uint8Array(11) [72, 101, 108, 108, 111, 32, 119, 111, 114, 108, 100]

Retrieving from a cache

To find an item in a cache, you can use the match method.

const response = await cache.match(request);
console.log(request, response);

If request is a string the browser converts it to a Request by calling new Request(request). The function returns a Promise that resolves to a Response if a matching entry is found, or undefined otherwise.

To determine if two Requests match, the browser uses more than just the URL. Two requests are considered different if they have different query strings, Vary headers, or HTTP methods (GET, POST, PUT, etc.).

You can ignore some or all of these things by passing an options object as a second parameter.

const options = {
  ignoreSearch: true,
  ignoreMethod: true,
  ignoreVary: true
};

const response = await cache.match(request, options);
// do something with the response

If more than one cached request matches then the one that was created first is returned. If you want to retrieve all matching responses, you can use cache.matchAll().

const options = {
  ignoreSearch: true,
  ignoreMethod: true,
  ignoreVary: true
};

const responses = await cache.matchAll(request, options);
console.log(`There are ${responses.length} matching responses.`);

As a shortcut you can search over all caches at once by using caches.match() instead of calling cache.match() for each cache.

Searching

The Cache API does not provide a way to search for requests or responses except for matching entries against a Response object. However, you can implement your own search using filtering or by creating an index.

Filtering

One way to implement your own search is to iterate over all entries and filter down to the ones that you want. Let's say that you want to find all items that have URLs ending with .png.

async function findImages() {
  // Get a list of all of the caches for this origin
  const cacheNames = await caches.keys();
  const result = [];

  for (const name of cacheNames) {
    // Open the cache
    const cache = await caches.open(name);

    // Get a list of entries. Each item is a Request object
    for (const request of await cache.keys()) {
      // If the request URL matches, add the response to the result
      if (request.url.endsWith('.png')) {
        result.push(await cache.match(request));
      }
    }
  }

  return result;
}

This way you can use any property of the Request and Response objects to filter the entries. Note that this is slow if you search over large sets of data.

Creating an index

The other way to implement your own search is to maintain a separate index of entries that can be searched and store the index in IndexedDB. Since this is the kind of operation that IndexedDB was designed for it has much better performance with large numbers of entries.

If you store the URL of the Request alongside the searchable properties then you can easily retrieve the correct cache entry after doing the search.

Deleting an item

To delete an item from a cache:

cache.delete(request);

Where request can be a Request or a URL string. This method also takes the same options object as cache.match, which allows you to delete multiple Request/Response pairs for the same URL.

cache.delete('/example/file.txt', {ignoreVary: true, ignoreSearch: true});

Deleting a cache

To delete a cache, call caches.delete(name). This function returns a Promise that resolves to true if the cache existed and was deleted, or false otherwise.

Thanks

Thanks to Mat Scales who wrote the original version of this article, which first appeared on WebFundamentals.