/Web APIs

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.


postMessage(message, transferables)



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

transferables Optional

A sequence of objects that are transferred with the message. 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).


Sending a message from a service worker to a client:

addEventListener("fetch", (event) => {
    (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.
        msg: "Hey I just got a fetch from you!",
        url: event.request.url,

Receiving that message:

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


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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