1. 9 Communication
    1. 9.1 The MessageEvent interface

9 Communication

9.1 The MessageEvent interface

MessageEvent

Support in all current engines.

Firefox4+Safari4+Chrome1+
Opera10.6+Edge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android4+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android11+

Messages in server-sent events, web sockets, cross-document messaging, channel messaging, and broadcast channels use the MessageEvent interface for their message events:

[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.data

MessageEvent/data

Support in all current engines.

Firefox4+Safari4+Chrome1+
OperaYesEdge79+
Edge (Legacy)12+Internet Explorer9+
Firefox AndroidYesSafari iOS3+Chrome AndroidYesWebView Android37+Samsung InternetYesOpera AndroidYes

Returns the data of the message.

event.origin

MessageEvent/origin

Support in all current engines.

Firefox4+Safari4+Chrome1+
OperaYesEdge79+
Edge (Legacy)12+Internet Explorer9+
Firefox Android4+Safari iOS3+Chrome AndroidYesWebView Android37+Samsung InternetYesOpera AndroidYes

Returns the origin of the message, for server-sent events and cross-document messaging.

event.lastEventId

MessageEvent/lastEventId

Support in all current engines.

Firefox4+Safari4+Chrome1+
OperaYesEdge79+
Edge (Legacy)17+Internet Explorer9+
Firefox AndroidYesSafari iOS3+Chrome AndroidYesWebView Android37+Samsung InternetYesOpera AndroidYes

Returns the last event ID string, for server-sent events.

event.source

MessageEvent/source

Support in all current engines.

Firefox55+Safari4+Chrome1+
Opera12.1+Edge79+
Edge (Legacy)12+Internet ExplorerNo
Firefox Android55+Safari iOS3+Chrome Android18+WebView Android37+Samsung Internet1.0+Opera Android12.1+

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.ports

MessageEvent/ports

Support in all current engines.

Firefox4+Safari4+Chrome1+
OperaYesEdge79+
Edge (Legacy)12+Internet Explorer9+
Firefox AndroidYesSafari iOS3+Chrome AndroidYesWebView Android37+Samsung InternetYesOpera AndroidYes

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.