Web/API/Client/postMessage

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

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

Parameters

message

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

undefined.

Examples

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.
    client.postMessage({
      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);
});

Specifications

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

Browser compatibility

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
postMessage Experimental'	Chrome Full support 45	Edge Full support ≤79	Firefox Full support 44 Notes' Full support 44 Notes' Notes' Extended Support Releases (ESR) before Firefox 78 ESR do not support service workers and the Push API.	IE No support No	Opera Full support 32	Safari No support No	WebView Android Full support 45	Chrome Android Full support 45	Firefox Android Full support 44	Opera Android Full support 32	Safari iOS No support No	Samsung Internet Android Full support 5.0

Legend

Full support  

Full support

No support  

No support

Experimental. Expect behavior to change in the future.'

Experimental. Expect behavior to change in the future.

See implementation notes.'

See implementation notes.

Client.postMessage() by Mozilla Contributors is licensed under CC-BY-SA 2.5.