TextTrackList.onchange

The TextTrackList property onchange is an event handler which is called when the change event occurs, indicating that a change has occurred on a TextTrack in the VideoTrackList.

TextTrackList.onchange = eventHandler;

This snippet establishes a handler for the change event that looks at each of the tracks in the list, calling a function to update the state of a user interface control that indicates the current state of the track.

var trackList = document.querySelector("video, audio").textTracks;

trackList.onchange = function(event) {
   .... /* do something */
};