How to calculate progress winxhr

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Javascript

Windows

Win8

WinJS

hi all
I am using following code to download video i am getting blob data successfully but i am not able to calculate progress percentage. please assist me.

WinJS.xhr({ url: vurl, responseType: "blob" })
.done(function complete(result) {

//do something with blob
},
function error(error) {
//log error
},
function progress(result) {
if (result.readyState > 2) {
//var totalBytes = result.getResponseHeader('Content-length');
//var dlBytes = result.responseText.length;
//var per = (totalBytes > 0) ? (Math.round((dlBytes / totalBytes) * 100) + "%") : (Math.round(dlBytes / 1024) + "K");
//$("#vmDownloadHeading").html("Downloading Media " + per);
}

});
if i use response type as text i can calculate progress by above code, for other response it will be undefined so how to calculate the progress?

regards
vinay