MessagePort: postMessage() method

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2015.

Note: This feature is available in Web Workers.

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

Syntax

js
postMessage(message)
postMessage(message, transfer)
postMessage(message, options)

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.

transfer Optional

An optional array of transferable objects to transfer ownership of. The ownership of these objects is given to the destination side and they are no longer usable on the sending side. These transferable objects should be attached to the message; otherwise they would be moved but not actually accessible on the receiving end.

options Optional

An optional object containing the following properties:

transfer Optional

Has the same meaning as the transfer parameter.

Return value

None (undefined).

Examples

In the following code block, you can see a new channel being created using the 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 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.

js
const channel = new MessageChannel();
const para = document.querySelector("p");

const ifr = document.querySelector("iframe");
const 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", (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

Specification
HTML
# dom-messageport-postmessage-dev

Browser compatibility

Report problems with this compatibility data on GitHub
desktopmobileserver
Chrome
Edge
Firefox
Opera
Safari
Chrome Android
Firefox for Android
Opera Android
Safari on iOS
Samsung Internet
WebView Android
WebView on iOS
Deno
Node.js
postMessage
options.includeUserActivation parameter
Non-standard

Legend

Tip: you can click/tap on a cell for more information.

Full support
Full support
Partial support
Partial support
No support
No support
Non-standard. Check cross-browser support before using.
See implementation notes.
Has more compatibility info.

See also