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chromium/third_party/blink/web_tests/external/wpt/html/semantics/embedded-content/media-elements/interfaces/TextTrackList/getTrackById.html 

<!doctype html>
<title>TextTrackList.getTrackById</title>
<script src=/resources/testharness.js></script>
<script src=/resources/testharnessreport.js></script>
<div id=log></div>
<script>
test(function(){
    var video = document.createElement('video');
    var track1 = video.addTextTrack('subtitles');
    var track2 = video.addTextTrack('subtitles');
    assert_equals(track1.id, '');
    assert_equals(track2.id, '');
    assert_equals(video.textTracks.getTrackById(''), track1);
    assert_equals(video.textTracks.getTrackById('fake-id'), null);
});
</script>