Client: postMessage() method

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since April 2018.

Note: This feature is only available in Service Workers.

The postMessage() method of the Client interface allows a service worker to send a message to a client (a Window, Worker, or SharedWorker). The message is received in the message event on navigator.serviceWorker.

Syntax

js
postMessage(message)
postMessage(message, transfer)
postMessage(message, options)

Parameters

message

The message to send to the client. This can be any structured-cloneable type.

Note: A service worker is not in the same agent cluster as its client, and therefore cannot share memory. SharedArrayBuffer objects, or buffer views backed by one, cannot be posted across agent clusters. Trying to do so will generate a messageerror event containing a DataCloneError DOMException on the receiving end.

transfer Optional

An optional array of transferable objects to transfer ownership of. The ownership of these objects is given to the destination side and they are no longer usable on the sending side. These transferable objects are not automatically sent; they must either be contained in the message or be accessible to the recipient via other means, such as MessagePort via MessageEvent.ports.

options Optional

An optional object containing the following properties:

transfer Optional

Has the same meaning as the transfer parameter.

Return value

None (undefined).

Examples

The code below sends a message from a service worker to a client. The client is fetched using the get() method on clients, which is a global in service worker scope.

js
addEventListener("fetch", (event) => {
  event.waitUntil(
    (async () => {
      // Exit early if we don't have access to the client.
      // Eg, if it's cross-origin.
      if (!event.clientId) return;

      // Get the client.
      const client = await self.clients.get(event.clientId);
      // Exit early if we don't get the client.
      // Eg, if it closed.
      if (!client) return;

      // Send a message to the client.
      client.postMessage({
        msg: "Hey I just got a fetch from you!",
        url: event.request.url,
      });
    })(),
  );
});

Receiving that message:

js
navigator.serviceWorker.addEventListener("message", (event) => {
  console.log(event.data.msg, event.data.url);
});

Specifications

Specification

Service Workers Nightly
# dom-client-postmessage-message-options

Browser compatibility

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This page was last modified on Apr 13, 2026 by MDN contributors.

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Client
Instance properties
Instance methods
1. postMessage()