MessagePort

The MessagePort interface of the Channel Messaging API represents one of the two ports of a MessageChannel, allowing sending of messages from one port and listening out for them arriving at the other.

Methods

Inherits methods from its parent, EventTarget

MessagePort.postMessage

Sends a message from the port, and optionally, transfers ownership of objects to other browsing contexts.

MessagePort.start

Starts the sending of messages queued on the port (only needed when using EventTarget.addEventListener; it is implied when using MessagePort.onmessage.)

MessagePort.close

Disconnects the port, so it is no longer active.

Event handlers

Inherits event handlers from its parent, EventTarget

MessagePort.onmessage

Is an EventListener that is called whenever an MessageEvent of type message is fired on the port — that is, when the port receives a message.

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 transfer MessageChannel.port2 (a MessagePort) to the IFrame using MessagePort.postMessage along with a message. The handleMessage handler then responds to a message being sent back from the IFrame (using MessagePort.onmessage), putting it into a paragraph.

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 'MessagePort' in that specification.	Living Standard	No difference from HTML5 Web Messaging.
HTML5 Web Messaging The definition of 'MessagePort' in that specification.	Recommendation	W3C version of the spec

Browser compatibility

See also

• Using channel messaging