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.

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 should be attached to the message; otherwise they would be moved but not actually accessible on the receiving end.

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
# dom-client-postmessage-message-options

Browser compatibility

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