index.md

title	slug	page-type	browser-compat
TrackEvent	Web/API/TrackEvent	web-api-interface	api.TrackEvent

{{APIRef("HTML DOM")}}

The TrackEvent interface, which is part of the HTML DOM specification, is used for events which represent changes to a set of available tracks on an HTML media element; these events are addtrack and removetrack.

It's important not to confuse TrackEvent with the {{domxref("RTCTrackEvent")}} interface, which is used for tracks which are part of an {{domxref("RTCPeerConnection")}}.

Events based on TrackEvent are always sent to one of the media track list types:

• Events involving video tracks are always sent to the {{domxref("VideoTrackList")}} found in {{domxref("HTMLMediaElement.videoTracks")}}
• Events involving audio tracks are always sent to the {{domxref("AudioTrackList")}} specified in {{domxref("HTMLMediaElement.audioTracks")}}
• Events affecting text tracks are sent to the {{domxref("TextTrackList")}} object indicated by {{domxref("HTMLMediaElement.textTracks")}}.

{{InheritanceDiagram}}

Constructor

• {{domxref("TrackEvent.TrackEvent", "TrackEvent()")}}
  • : Creates and initializes a new TrackEvent object with the event type specified, as well as optional additional properties.

Instance properties

TrackEvent is based on {{domxref("Event")}}, so properties of Event are also available on TrackEvent objects.

• {{domxref("TrackEvent.track", "track")}} {{ReadOnlyInline}}
  • : The DOM track object the event is in reference to. If not null, this is always an object of one of the media track types: {{domxref("AudioTrack")}}, {{domxref("VideoTrack")}}, or {{domxref("TextTrack")}}).

Instance methods

TrackEvent has no methods of its own; however, it is based on {{domxref("Event")}}, so it provides the methods available on Event objects.

Example

This example sets up a function, handleTrackEvent(), which is called for any addtrack or removetrack event on the first {{HTMLElement("video")}} element found in the document.

const videoElem = document.querySelector("video");

videoElem.videoTracks.addEventListener("addtrack", handleTrackEvent, false);
videoElem.videoTracks.addEventListener("removetrack", handleTrackEvent, false);
videoElem.audioTracks.addEventListener("addtrack", handleTrackEvent, false);
videoElem.audioTracks.addEventListener("removetrack", handleTrackEvent, false);
videoElem.textTracks.addEventListener("addtrack", handleTrackEvent, false);
videoElem.textTracks.addEventListener("removetrack", handleTrackEvent, false);

function handleTrackEvent(event) {
  let trackKind;

  if (event.target instanceof VideoTrackList) {
    trackKind = "video";
  } else if (event.target instanceof AudioTrackList) {
    trackKind = "audio";
  } else if (event.target instanceof TextTrackList) {
    trackKind = "text";
  } else {
    trackKind = "unknown";
  }

  switch (event.type) {
    case "addtrack":
      console.log(`Added a ${trackKind} track`);
      break;
    case "removetrack":
      console.log(`Removed a ${trackKind} track`);
      break;
  }
}

The event handler uses the JavaScript instanceof operator to determine which type of track the event occurred on, then outputs to console a message indicating what kind of track it is and whether it's being added to or removed from the element.

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}