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.

Note: This feature is available in Web Workers

Methods

Inherits methods from its parent, EventTarget

postMessage()

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

start()

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

close()

Disconnects the port, so it is no longer active.

Event handlers

Inherits event handlers from its parent, EventTarget

onmessage

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

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() 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 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

Specification
HTML Standard (HTML)
# message-ports

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
MessagePort
4
12
41
10
10.6
5
≤37
18
41
11
4.2
1.0
close
4
12
41
10
10.6
5
≤37
18
41
11
4.2
1.0
message_event
4
12
Yes
10
10.6
5
≤37
18
No
11.5
4.2
1.0
messageerror_event
60
18
57
No
47
No
60
60
57
47
No
8.0
onmessage
4
12
41
10
10.6
5
≤37
18
41
11
4.2
1.0
onmessageerror
60
18
57
No
47
No
60
60
57
44
No
8.0
postMessage
4
12
41
10
10.6
5
≤37
18
41
11
4.2
1.0
start
4
12
41
10
10.6
5
≤37
18
41
11
4.2
1.0
worker_support
4
12
41
10
≤15
5
≤37
18
41
≤14
5
1.0

See also

© 2005–2021 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/API/MessagePort