MessagePort: message event

Note: This feature is available in Web Workers.

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

This event is not cancellable and does not bubble.


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

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

onmessage = (event) => {};

Event type

Event properties

This interface also inherits properties from its parent, Event. Read only

The data sent by the message emitter.

MessageEvent.origin Read only

A string representing the origin of the message emitter.

MessageEvent.lastEventId Read only

A string representing a unique ID for the event.

MessageEvent.source Read only

A MessageEventSource (which can be a WindowProxy, MessagePort, or ServiceWorker object) representing the message emitter.

MessageEvent.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).


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 =[1];
const targetOrigin = "";

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

channelMessageButton.addEventListener("click", () => {

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 =;


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 =;


HTML Standard
# event-message
HTML Standard
# handler-messageport-onmessage

Browser compatibility

BCD tables only load in the browser

See also