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---
title: MessagePort
slug: Web/API/MessagePort
tags:
- API
- Channel messaging
- HTML5
- Interface
- MessagePort
- NeedsTranslation
- Reference
- TopicStub
translation_of: Web/API/MessagePort
---
<p>{{APIRef("HTML DOM")}}</p>
<p>The <strong><code>MessagePort</code></strong> interface of the <a href="/en-US/docs/Web/API/Channel_Messaging_API">Channel Messaging API</a> represents one of the two ports of a {{domxref("MessageChannel")}}, allowing messages to be sent from one port and listening out for them arriving at the other.</p>
<p>{{AvailableInWorkers}}</p>
<h2 id="Methods">Methods</h2>
<p><em>Inherits methods from its parent, {{domxref("EventTarget")}}</em></p>
<dl>
<dt>{{domxref("MessagePort.postMessage","postMessage()")}}</dt>
<dd>Sends a message from the port, and optionally, transfers ownership of objects to other browsing contexts.</dd>
<dt>{{domxref("MessagePort.start","start()")}}</dt>
<dd>Starts the sending of messages queued on the port (only needed when using {{domxref("EventTarget.addEventListener")}}; it is implied when using {{domxref("MessagePort.onmessage")}}.)</dd>
<dt>{{domxref("MessagePort.close","close()")}}</dt>
<dd>Disconnects the port, so it is no longer active.</dd>
</dl>
<h2 id="Event_handlers">Event handlers</h2>
<p><em>Inherits event handlers from its parent, {{domxref("EventTarget")}}</em></p>
<dl>
<dt>{{domxref("MessagePort.onmessage","onmessage")}}</dt>
<dd>An {{domxref("EventListener")}} called when {{domxref("MessageEvent")}} of type <code>message</code> is fired on the port—that is, when the port receives a message.</dd>
<dt>{{domxref("MessagePort.onmessageerror","onmessageerror")}}</dt>
<dd>An {{domxref("EventListener")}} called when a {{domxref("MessageEvent")}} of type {{domxref("MessageError")}} is fired—that is, when it receives a message that cannot be deserialized.</dd>
</dl>
<h2 id="Events">Events</h2>
<dl>
<dt>{{domxref("MessagePort.message_event","message")}}</dt>
<dd>Fired when a <code>MessagePort</code> object receives a message.<br>
Also available via the {{domxref("MessagePort.onmessage","onmessage")}} property.</dd>
<dt>{{domxref("MessagePort.messageerror_event","messageerror")}}</dt>
<dd>Fired when a <code>MessagePort</code> object receives a message that can't be deserialized.<br>
Also available via the {{domxref("MessagePort.onmessageerror","onmessageerror")}} property.</dd>
</dl>
<h2 id="Example">Example</h2>
<p>In the following example, you can see a new channel being created using the {{domxref("MessageChannel.MessageChannel","MessageChannel()")}} constructor.</p>
<p>When the IFrame has loaded, we register an {{domxref("MessagePort.onmessage","onmessage")}} handler for {{domxref("MessageChannel.port1")}} and transfer {{domxref("MessageChannel.port2")}} to the IFrame using the {{domxref("window.postMessage")}} method along with a message.</p>
<p>When a message is received back from the IFrame, the <code>onMessage</code> function simply outputs the message to a paragraph.</p>
<pre>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;
}
</pre>
<p>For a full working example, see our <a class="external external-icon" href="https://github.com/mdn/dom-examples/tree/master/channel-messaging-basic">channel messaging basic demo</a> on Github (<a class="external external-icon" href="https://mdn.github.io/dom-examples/channel-messaging-basic/">run it live too</a>).</p>
<h2 id="Specifications">Specifications</h2>
<table class="standard-table">
<tbody>
<tr>
<th scope="col">Specification</th>
<th scope="col">Status</th>
<th scope="col">Comment</th>
</tr>
<tr>
<td>{{SpecName('HTML WHATWG', 'web-messaging.html#message-ports','Message ports')}}</td>
<td>{{Spec2('HTML WHATWG')}}</td>
<td></td>
</tr>
</tbody>
</table>
<h2 id="Browser_compatibility">Browser compatibility</h2>
<div>
<p>{{Compat("api.MessagePort")}}</p>
</div>
<h2 id="See_also">See also</h2>
<ul>
<li><a href="/en-US/docs/Web/API/Channel_Messaging_API/Using_channel_messaging">Using channel messaging</a></li>
</ul>
|
