video.js

vjs.MediaTechController.prototype.emulateTextTracks = function() {
  var player = this.player_,
      textTracksChanges,
      tracks,
      script;

  if (!window['WebVTT']) {
    script = document.createElement('script');
    script.src = player.options()['vtt.js'] || '../node_modules/vtt.js/dist/vtt.js';
    player.el().appendChild(script);
    window['WebVTT'] = true;
  }

  tracks = this.textTracks();
  if (!tracks) {
    return;
  }

  // ...
}