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.

Syntax

port.postMessage(message, transferList);

Returns

Void.

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.

Example

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 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('Hello from the main page!', '*', [channel.port2]);
}

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

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

Specifications

Specification	Status	Comment
WHATWG HTML Living Standard The definition of 'postMessage()' in that specification.	Living Standard	No difference from HTML5 Web Messaging.
HTML5 Web Messaging The definition of 'postMessage()' in that specification.	Recommendation	W3C version of the spec

Browser compatibility

See also

• Using channel messaging