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The onmessage event handler of the MessagePort interface is an EventListener, called whenever an MessageEvent of type message is fired on the port — that is, when the port receives a message.

Note: This feature is available in Web Workers.

Syntax

channel.onmessage = function() { ... };

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 pass MessageChannel.port2 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 onmessage, putting it into a paragraph — MessageChannel.port1 is listened to, to check when the message arrives.

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

Specification Status Comment
HTML Living Standard
The definition of 'onmessage' in that specification.
Living Standard  

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 4 (Yes) (Yes) 10.0 10.6 5
Available in workers (Yes) ? 41 (41) (Yes) (Yes) (Yes)
Feature Android Chrome for Android Edge Firefox Mobile (Gecko) Firefox OS (Gecko) IE Phone Opera Mobile Safari Mobile
Basic support 4.4 4 (Yes) No support No support 10.0 11.5 5.1
Available in workers (Yes) (Yes) ? 41.0 (41) (Yes) (Yes) (Yes) (Yes)

See also

Document Tags and Contributors

 Last updated by: DomenicDenicola,