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 MessageChannel.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Disconnects the port, so it is no longer active.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

Browser compatibility

Compatibility notes

Channel messaging support for Firefox is close; MessagePort is already being used in shared workers. See bug 952139 for progress.

See also

Using channel messaging