Checking download state in chrome

I have a web application deployed as a .WAR file.

I'm trying to implement a download feature, however I was wondering if it's possible to perform checking to see if Chrome is downloading a file currently.I've found the following page/method : http://ift.tt/1e698eQ

chrome.downloads.onCreated.addListener(function callback)

however it doesn't appear to work as downloads is always undefined. Is there an alternative way to check the download state ?