HTML
Living Standard — Last Updated 23 October 2025
Living Standard — Last Updated 23 October 2025
The WebSocket interface used to be defined here. It is now defined in WebSockets. [WEBSOCKETS]
MessageEvent interfaceSupport in all current engines.
Messages in server-sent events, cross-document messaging, channel messaging, broadcast channels, and WebSockets use the MessageEvent interface for their message events: [WEBSOCKETS]
[Exposed=(Window,Worker,AudioWorklet)]
interface MessageEvent : Event {
constructor(DOMString type, optional MessageEventInit eventInitDict = {});
readonly attribute any data;
readonly attribute USVString origin;
readonly attribute DOMString lastEventId;
readonly attribute MessageEventSource? source;
readonly attribute FrozenArray<MessagePort> ports;
undefined initMessageEvent(DOMString type, optional boolean bubbles = false, optional boolean cancelable = false, optional any data = null, optional USVString origin = "", optional DOMString lastEventId = "", optional MessageEventSource? source = null, optional sequence<MessagePort> ports = []);
};
dictionary MessageEventInit : EventInit {
any data = null;
USVString origin = "";
DOMString lastEventId = "";
MessageEventSource? source = null;
sequence<MessagePort> ports = [];
};
typedef (WindowProxy or MessagePort or ServiceWorker) MessageEventSource;event.dataSupport in all current engines.
Returns the data of the message.
event.originSupport in all current engines.
Returns the origin of the message, for server-sent events and cross-document messaging.
event.lastEventIdSupport in all current engines.
Returns the last event ID string, for server-sent events.
event.sourceSupport in all current engines.
Returns the WindowProxy of the source window, for cross-document messaging, and the MessagePort being attached, in the connect event fired at SharedWorkerGlobalScope objects.
event.portsSupport in all current engines.
Returns the MessagePort array sent with the message, for cross-document messaging and channel messaging.
The data attribute must return the value it was initialized to. It represents the message being sent.
The origin attribute must return the value it was initialized to. It represents, in server-sent events and cross-document messaging, the origin of the document that sent the message (typically the scheme, hostname, and port of the document, but not its path or fragment).
The lastEventId attribute must return the value it was initialized to. It represents, in server-sent events, the last event ID string of the event source.
The source attribute must return the value it was initialized to. It represents, in cross-document messaging, the WindowProxy of the browsing context of the Window object from which the message came; and in the connect events used by shared workers, the newly connecting MessagePort.
The ports attribute must return the value it was initialized to. It represents, in cross-document messaging and channel messaging, the MessagePort array being sent.
The initMessageEvent(type, bubbles, cancelable, data, origin, lastEventId, source, ports) method must initialize the event in a manner analogous to the similarly-named initEvent() method. [DOM]
Various APIs (e.g., WebSocket, EventSource) use the MessageEvent interface for their message event without using the MessagePort API.