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This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the specification changes.

The Client.postMessage() method of the Client interface allows a service worker client to send a message to a ServiceWorker.

Note: Messages sent to service worker contexts (e.g. as the event object of onmessage are represented by MessageEvent objects in modern browsers, for consistency with other web messaging features. (They used to be represented by ServiceWorkerMessageEvent objects, which have now been deprecated.)


Client.postMessage(message[, transfer]);




The message to send to the service worker.
transfer Optional
A transferable object such as, for example, a reference to a port.


This code snippet is taken from the service worker post-message sample (see post-message live.) The code sends message data and transfers the port to the service worker so that it can reply via Client.postMessage().

The message is wrapped in a promise that resolves if the response doesn't contain an error and rejects with the error.

function sendMessage(message) {
  return new Promise(function(resolve, reject) {
     var messageChannel = new MessageChannel();
     messageChannel.port1.onmessage = function(event) {
       if ( {
       } else {
    navigator.serviceWorker.controller.postMessage(message, [messageChannel.port2]);


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

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 45.0[1] 44.0 (44.0)[2] No support ? No support
Feature Android Android Webview Firefox Mobile (Gecko) Firefox OS IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support No support No support 44.0 (44.0) ? No support ? No support 45.0 [1]

Document Tags and Contributors

 Contributors to this page: chrisdavidmills, jpmedley, fscholz, kscarfone
 Last updated by: chrisdavidmills,