MessagePort: message event

The message event is fired on a MessagePort object when a message arrives on that channel.

Examples

Suppose a script creates a MessageChannel and sends one of the ports to a different browsing context, such as another <iframe>, using code like this:

const channel = new MessageChannel();
const myPort = channel.port1;
const targetFrame = window.top.frames[1];
const targetOrigin = 'https://example.org';

const messageControl = document.querySelector('#message');
const channelMessageButton = document.querySelector('#channel-message');

channelMessageButton.addEventListener('click', () => {
    myPort.postMessage(messageControl.value);
})

targetFrame.postMessage('init', targetOrigin, [channel.port2]);

The target can receive the port and start listening for messages on it using code like this:

window.addEventListener('message', (event) => {
    const myPort = event.ports[0];

    myPort.addEventListener('message', (event) => {
        received.textContent = event.data;
    });

    myPort.start();
});

Note that the listener must call MessagePort.start() before any messages will be delivered to this port. This is only needed when using the addEventListener() method: if the receiver uses onmessage instead, start() is called implicitly:

window.addEventListener('message', (event) => {
    const myPort = event.ports[0];

    myPort.onmessage = (event) => {
        received.textContent = event.data;
    };
});

Specifications

Browser compatibility

See also

• Related events: messageerror.
• Using channel messaging