MessagePort

The MessagePort interface of the Channel Messaging API represents one of the two ports of a MessageChannel, allowing messages to be sent 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Disconnects the port, so it is no longer active.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

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

MessagePort.onmessageerror

An EventListener called when a MessageEvent of type MessageError is fired—that is, when it receives a message that cannot be deserialized.

Events

message

Fired when a MessagePort object receives a message.
Also available via the onmessage property.

messageerror

Fired when a MessagePort object receives a message that can't be deserialized.
Also available via the onmessageerror property.

Example

In the following example, you can see a new channel being created using the MessageChannel.MessageChannel constructor.

When the IFrame has loaded, we register an onmessage handler for  MessageChannel.port1 and transfer MessageChannel.port2 to the IFrame using the window.postMessage method along with a message.

When a message is received back from the IFrame, the onMessage function simply outputs the message to a paragraph.

var channel = new MessageChannel();
var output = document.querySelector('.output');
var iframe = document.querySelector('iframe');

// Wait for the iframe to load
iframe.addEventListener("load", onLoad);

function onLoad() {
  // Listen for messages on port1
  channel.port1.onmessage = onMessage;

  // Transfer port2 to the iframe
  iframe.contentWindow.postMessage('Hello from the main page!', '*', [channel.port2]);
}


// Handle messages received on port1
function onMessage(e) {
  output.innerHTML = e.data;
}

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

Specifications

Browser compatibility

See also

• Using channel messaging