Client: postMessage() method

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, options)
postMessage(message, transferables)

Parameters

message

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

options Optional

An optional object containing a transfer field with an 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.

transferables 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.

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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