The postMessage() method of the Client interface allows a service worker to send a message to a client (a WindowWorker, or SharedWorker). The message is received in the "message" event on navigator.serviceWorker.


client.postMessage(message[, transfer]);
client.postMessage(message[, { transfer }]);


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



Sending a message from a service worker to a client:

addEventListener('fetch', event => {
  event.waitUntil(async function() {
    // 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 => {


Specification Status Comment
Service Workers
The definition of 'postMessage()' in that specification.
Working Draft Initial definition.

Browser compatibility

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