Find duration of media file asynchronously in JavaScript

gistfile1.js

function getDuration(fileURL, cb){
  var a = document.createElement('audio');
  a.src = fileURL;
  a.preload = 'metadata';
  document.body.appendChild(a);
  a.addEventListener('loadedmetadata', function(evt){
    var d = evt.target.duration;
    cb(d);
  }, false);
}
// Notes:
// Does not require full file to load, and can be retrieved async.
// Example:
// getDuration('http://media.tts-api.com/c99414d17e9831923df12bda67c3b468da7b9619.mp3', function(d){alert(d);}