Download Attribute Overriding onClick <a>

I am using the download attribute to trigger a forced download of a song, but it seems to be overriding the onClick event. I can only do one or the other.

<output></output>
 <input onclick="window.close()" type="button" value="Cancel"/>
 
 <script>
 var a = document.createElement("a");
 a.download = song + ".mp3"
 a.href = url
 a.onclick = function(e) {
     var notification = webkitNotifications.createHTMLNotification(
         'notification2.html'
     );
     notification.show();
     window.close();
 }
 document.getElementsByTagName("output")[0].appendChild(a);
 document.getElementsByTagName("a")[0].innerHTML = "Download"
 </script>
 

How do I both trigger the download, and fire the onClick event?

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