BroadcastChannel: message event

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

Syntax

Use the event name in methods like addEventListener(), or set an event handler property.

addEventListener("message", (event) => { })
onmessage = (event) => { }

Event type

A MessageEvent. Inherits from Event.

Event properties

In addition to the properties listed below, properties from the parent interface, Event, are available.

data Read only: The data sent by the message emitter.
origin Read only: A string representing the origin of the message emitter.
lastEventId Read only: A string representing a unique ID for the event.
source Read only: A message event source, which is either a WindowProxy, a MessagePort, or a ServiceWorker object representing the message emitter.
ports Read only: An array of MessagePort objects representing the ports associated with the channel the message is being sent through (where appropriate, e.g. in channel messaging or when sending a message to a shared worker).

In this example there's a "sender" <iframe> that broadcasts the contents of a <textarea> when the user clicks a button. There are two "receiver" iframes that listen to the broadcast message and write the result into a <div> element.

Sender

<h1>Sender</h1>
<label for="message">Type a message to broadcast:</label><br />
<textarea id="message" name="message" rows="1" cols="40">Hello</textarea>
<button id="broadcast-message" type="button">Broadcast message</button>

body {
     border: 1px solid black;
     padding: .5rem;
     height: 150px;
     font-family: "Fira Sans", sans-serif;
}

h1 {
    font: 1.6em "Fira Sans", sans-serif;
    margin-bottom: 1rem;
}

textarea {
    padding: .2rem;
}

label, br {
    margin: .5rem 0;
}

button {
    vertical-align: top;
    height: 1.5rem;
}

const channel = new BroadcastChannel("example-channel");
const messageControl = document.querySelector("#message");
const broadcastMessageButton = document.querySelector("#broadcast-message");

broadcastMessageButton.addEventListener("click", () => {
  channel.postMessage(messageControl.value);
});

Receiver 1

<h1>Receiver 1</h1>
<div id="received"></div>

body {
    border: 1px solid black;
    padding: .5rem;
    height: 100px;
    font-family: "Fira Sans", sans-serif;
}

h1 {
    font: 1.6em "Fira Sans",
    sans-serif; margin-bottom: 1rem;
}

const channel = new BroadcastChannel("example-channel");
channel.addEventListener("message", (event) => {
  received.textContent = event.data;
});

Receiver 2

<h1>Receiver 2</h1>
<div id="received"></div>

body {
    border: 1px solid black;
    padding: .5rem;
    height: 100px;
    font-family: "Fira Sans", sans-serif;
}

h1 {
    font: 1.6em "Fira Sans", sans-serif;
    margin-bottom: 1rem;
}

const channel = new BroadcastChannel("example-channel");
channel.addEventListener("message", (event) => {
  received.textContent = event.data;
});

Result

Specifications

Specification
HTML Standard # event-message
HTML Standard # handler-broadcastchannel-onmessage

Browser compatibility

BCD tables only load in the browser