The Clients API: a service worker's access to client objects

In one line: The Clients interface, accessed as self.clients inside a service worker, provides access to Client objects (windows, workers) matching the query options passed in — with matchAll({ includeUncontrolled: true }), this can include clients that share the same storage key/origin but are not controlled by, or associated with the registration of, this service worker.

This feature is only available in service workers.

Accessing it

// Inside a service worker
self.clients;

Methods

Method	Returns	Purpose
Clients.get(id)	Promise<Client | undefined>	Returns a Client matching a given id, or undefined if no client matches.
Clients.matchAll(options)	Promise<Array<Client>>	Returns an array of Client objects. By default only controlled clients are returned; an options argument allows control over the types of clients returned, including uncontrolled ones.
Clients.openWindow(url)	Promise<WindowClient | null>	Opens a new browser window for a given URL, resolving to null if the platform cannot return a same-origin WindowClient for it.
Clients.claim()	Promise<void>	Allows an active service worker to set itself as the controller for all clients within its scope.

Example

Finding an already-open chat window, focusing it, or opening a new one, then messaging it:

addEventListener("notificationclick", (event) => {
  event.waitUntil(
    (async () => {
      const allClients = await clients.matchAll({
        includeUncontrolled: true,
      });

      let chatClient;

      // Let's see if we already have a chat window open:
      for (const client of allClients) {
        const url = new URL(client.url);

        if (url.pathname === "/chat/") {
          // Excellent, let's use it!
          client.focus();
          chatClient = client;
          break;
        }
      }

      // If we didn't find an existing chat window,
      // open a new one:
      chatClient ??= await clients.openWindow("/chat/");

      // Message the client:
      chatClient.postMessage("New chat messages!");
    })(),
  );
});

Browser support

The Clients interface is Baseline widely available: it is well established and works across many devices and browser versions, and has been available across browsers since April 2018.

Practical checklist

• Use clients.matchAll() with includeUncontrolled when a service worker needs to find windows it does not yet control — by default matchAll() only returns controlled clients.
• Handle clients.get(id) resolving to undefined and clients.openWindow(url) resolving to null when no matching client can be returned.
• Call clients.claim() in the service worker’s activate handler when it should take control of existing clients immediately, rather than waiting for the next navigation.
• Use clients.openWindow() only in response to a user gesture such as a notification click, matching the pattern shown above.

Cross-references

• Navigation preload — another service worker capability exposed via a similarly scoped API