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, transferables)

Parameters

message

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).

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

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

Browser compatibility

BCD tables only load in the browser