AudioTrackList.length

A number indicating how many audio tracks are included in the AudioTrackList. Each track can be accessed by treating the AudioTrackList as an array of objects of type AudioTrack.

This snippet gets the number of audio tracks in the first <video> element found in the DOM by querySelector().

var videoElem = document.querySelector("video");
var numAudioTracks = 0;

if (videoElem.audioTracks) {
  numAudioTracks = videoElem.audioTracks.length;
}

Note that this sample checks to be sure HTMLMediaElement.audioTracks is defined, to avoid failing on browsers without support for AudioTrack.