BroadcastChannel: message event

{{APIRef("BroadCastChannel API")}}{{AvailableInWorkers}}

The message event of the {{domxref("BroadcastChannel")}} interface fires when a message arrives on that channel.

Syntax

Use the event name in methods like {{domxref("EventTarget.addEventListener", "addEventListener()")}}, or set an event handler property.

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

onmessage = (event) => { }

Event type

A {{domxref("MessageEvent")}}. Inherits from {{domxref("Event")}}.

{{InheritanceDiagram("MessageEvent")}}

Event properties

In addition to the properties listed below, properties from the parent interface, {{domxref("Event")}}, are available.

• {{domxref("MessageEvent.data", "data")}} {{ReadOnlyInline}}
  • : The data sent by the message emitter.
• {{domxref("MessageEvent.origin", "origin")}} {{ReadOnlyInline}}
  • : A string representing the origin of the message emitter.
• {{domxref("MessageEvent.lastEventId", "lastEventId")}} {{ReadOnlyInline}}
  • : A string representing a unique ID for the event.
• {{domxref("MessageEvent.source", "source")}} {{ReadOnlyInline}}
  • : A message event source, which is either a {{glossary("WindowProxy")}}, a {{domxref("MessagePort")}}, or a {{domxref("ServiceWorker")}} object representing the message emitter.
• {{domxref("MessageEvent.ports", "ports")}} {{ReadOnlyInline}}
  • : An array of {{domxref("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).

Examples

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

Sender

<h1>Sender</h1>
<label for="message">Type a message to broadcast:</label>

<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: 0.5rem;
  height: 150px;
  font-family: "Fira Sans", sans-serif;
}

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

textarea {
  padding: 0.2rem;
}

label,
br {
  margin: 0.5rem 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: 0.5rem;
  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: 0.5rem;
  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

{{ EmbedLiveSample('Sender', '100%', 220) }}

{{ EmbedLiveSample('Receiver_1', '100%', 160) }}

{{ EmbedLiveSample('Receiver_2', '100%', 160) }}

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• Related events: {{domxref("BroadcastChannel/messageerror_event", "messageerror")}}.