MediaStream.onaddtrack

The MediaStream.onaddtrack property is an EventHandler which specifies a function to be called when the addtrack event occurs on a MediaStream instance. This happens when a new track of any kind is added to the media stream. This event is fired when the browser adds a track to the stream (such as when a RTCPeerConnection is renegotiated or a stream being captured using HTMLMediaElement.captureStream() gets a new set of tracks because the media element being captured loaded a new source.

The addtrack event does not get fired when JavaScript code explicitly adds tracks to the stream (by calling addTrack()).

MediaStream.onaddtrack = eventHandler;

This should be set to a function which you provide that accepts as input a MediaStreamTrackEvent object representing the addtrack event which has occurred. The MediaStreamTrack representing the track which was added is specified in the event's track property.

This example adds a listener which, when a new track is added to the stream, appends a new item to a list of tracks; the new item shows the track's kind ("audio" or "video") and label.

stream.onaddtrack = function(event) {
  let trackList = document.getElementById("tracks");
  let label = document.createElement("li");

  label.innerHTML = event.track.kind + ": " + event.track.label;
  trackList.appendChild(label);
};

• The addtrack event and its type, MediaStreamTrackEvent.