dom / latest / messageport / postmessage.html /

MessagePort.postMessage()

The postMessage() method of the MessagePort interface sends a message from the port, and optionally, transfers ownership of objects to other browsing contexts.

Note: This feature is available in Web Workers

Syntax

postMessage(message, transferList)

Return value

Parameters

message

The message you want to send through the channel. This can be of any basic data type. Multiple data items can be sent as an array.

transferList Optional

Transferable Objects to be transferred — these objects have their ownership transferred to the receiving browsing context, so are no longer usable by the sending browsing context.

Examples

In the following code block, you can see a new channel being created using the MessageChannel.MessageChannel constructor. When the IFrame has loaded, we pass MessageChannel.port2 to the IFrame using window.postMessage along with a message. The iframe receives the message, and sends a message back on the MessageChannel using MessageChannel.postMessage. The handleMessage handler then responds to a message being sent back from the iframe using onmessage, putting it into a paragraph — MessageChannel.port1 is listened to, to check when the message arrives.

var channel = new MessageChannel();
var para = document.querySelector('p');

var ifr = document.querySelector('iframe');
var otherWindow = ifr.contentWindow;

ifr.addEventListener("load", iframeLoaded, false);

function iframeLoaded() {
  otherWindow.postMessage('Transferring message port', '*', [channel.port2]);
}

channel.port1.onmessage = handleMessage;
function handleMessage(e) {
  para.innerHTML = e.data;
}

// in the iframe...

window.addEventListener('message', function (event) {
  const messagePort = event.ports?.[0];
  messagePort.postMessage("Hello from the iframe!");
});

For a full working example, see our channel messaging basic demo on GitHub (run it live too).

Specifications

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
postMessage
1
12
41
10
10.6
5
4.4
18
41
11
4.2
1.0

See also

© 2005–2021 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/API/MessagePort/postMessage