Windows 8 JavaScript Metro Application–Getting Started

/// <resource type="javascript" src="//Microsoft.WinJS.0.6/js/base.js" shared="true" />
/// <resource type="javascript" src="//Microsoft.WinJS.0.6/js/ui.js" shared="true" />

(function () {
    "use strict";
    var folder = Windows.Storage.ApplicationData.current.localFolder;

    function download(imgUrl, imgName) {
        return WinJS.xhr({ url: imgUrl, responseType: "blob" }).then(function (result) {
            var blob = result.response;
            return folder.createFileAsync(imgName, Windows.Storage.CreationCollisionOption.replaceExisting).then(function (file) {
                // Open the returned file in order to copy the data
                return file.openAsync(Windows.Storage.FileAccessMode.readWrite).then(function (stream) {
                    return Windows.Storage.Streams.RandomAccessStream.copyAsync(blob.msDetachStream(), stream).then(function () {
                        // Copy the stream from the blob to the File stream
                        return stream.flushAsync().then(function () {
                            stream.close();
                        });
                    });
                });
            });
        }, function (e) {
            var msg = new Windows.UI.Popups.MessageDialog(e.message);
            msg.showAsync();
        });
    }

    function fileExists(fileName) {
        return folder.getFileAsync(fileName).then(function (file) {
            return file;
        }, function (err) {
            return null;
        });
    }

    WinJS.Namespace.define('imgDownloader', {
        download: download,
        fileExists: fileExists
    });
})();