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Client: postMessage() method

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

Sending a message from a service worker to a client:

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 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

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
postMessage 40 17 44 No 27 11.1 40 40 44 27 11.3 4.0

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https://developer.mozilla.org/en-US/docs/Web/API/Client/postMessage