---
title: MessagePort.onmessage
slug: Web/API/MessagePort/onmessage
translation_of: Web/API/MessagePort/onmessage
---
<div>{{APIRef("HTML DOM")}}</div>

<p> {{domxref("MessagePort")}} 接口的 <code><strong>onmessage </strong></code>事件处理程序是一个{{domxref("EventListener")}}, 当在端口上触发一个类型为 <strong>message</strong> 的{{domxref("MessageEvent")}} 时被调用 - 即,当端口接收到消息时。</p>

<p>{{AvailableInWorkers}}</p>

<h2 id="Syntax">Syntax</h2>

<pre class="syntaxbox">channel.onmessage = function() { ... };</pre>

<h2 id="Example">Example</h2>

<p>In the following code block, you can see a new channel being created using the {{domxref("MessageChannel()", "MessageChannel.MessageChannel")}} constructor. When the IFrame has loaded, we pass {{domxref("MessageChannel.port2")}} to the IFrame using {{domxref("MessagePort.postMessage")}} along with a message. The <code>handleMessage</code> handler then responds to a message being sent back from the IFrame using <code>onmessage</code>, putting it into a paragraph — {{domxref("MessageChannel.port1")}} is listened to, to check when the message arrives.</p>

<pre class="brush: js">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;
}   </pre>

<p>For a full working example, see our <a class="external external-icon" href="https://github.com/mdn/channel-messaging-basic-demo">channel messaging basic demo</a> on Github (<a class="external external-icon" href="http://mdn.github.io/channel-messaging-basic-demo/">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', '#handler-messageport-onmessage','onmessage')}}</td>
   <td>{{Spec2('HTML WHATWG')}}</td>
   <td>No difference from {{SpecName("HTML5 Web Messaging")}}.</td>
  </tr>
  <tr>
   <td>{{SpecName('HTML5 Web Messaging', '#handler-messageport-onmessage','onmessage')}}</td>
   <td>{{Spec2('HTML5 Web Messaging')}}</td>
   <td>W3C version of the spec</td>
  </tr>
 </tbody>
</table>

<h2 id="Browser_compatibility">Browser compatibility</h2>

{{Compat("api.MessagePort.onmessage")}}

<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>