Fullscreen function javascript mediaplayer -

i want incorporate fullscreen function (with button) in videoplayer i'm making javascript. code:

html:

<div id="buttonbar" style="display: none;")> <button id="fullscreen" title="full screen">[oo]</button> 

this creates button fullscreen functon. in .js have got following code:

var fullscreen = function() { 'use strict'; if (video.requestfullscreen) { video.requestfullscreen(); } else if (video.webkitrequestfullscreen) { video.webkitrequestfullscreen();  document.getelementbyid("fullscreen").addeventlistener("click", function () { video.requestfullscreen(); alert("goes fullscreen"); }, false); 

somehow button doesn't respond when click button, can't figure out why. how can fix this?