This code snippet allows you to store data in winJS local folder.

 var localFolder = Windows.Storage.ApplicationData.current.localFolder; //--- gets the local folder
            localFolder.createFileAsync("TestLog.txt", Windows.Storage.CreationCollisionOption.openIfExists).done(function (testLog) {
                //do your operation on the file testLog
})

Windows.Storage.CreationCollisionOption.openIfExists -> option to specify what to do when file is present.
If present then open the existing file or create a new file