Client.postMessage()

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.

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 Workers
The definition of 'postMessage()' in that specification.
Working Draft Initial definition.

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
postMessage
Experimental
Chrome Full support 45Edge Full support ≤79Firefox 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 NoOpera Full support 32Safari No support NoWebView Android Full support 45Chrome Android Full support 45Firefox Android Full support 44Opera Android Full support 32Safari iOS No support NoSamsung 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.