FileIO.WriteBufferAsync | writeBufferAsync method

Windows.Storage.FileIO.writeBufferAsync(file, buffer).done( /* Your success and error handlers */ );

if (file !== null) {
    // Create buffer
    var memoryStream = new Windows.Storage.Streams.InMemoryRandomAccessStream();
    var dataWriter = new Windows.Storage.Streams.DataWriter(memoryStream);
    dataWriter.writeString("Swift as a shadow");
    var buffer = dataWriter.detachBuffer();
    dataWriter.close();

    // Write buffer to file
    Windows.Storage.FileIO.writeBufferAsync(file, buffer).done(function () {
        // Perform additional tasks after file is written
    },
    // Handle errors with an error function
    function (error) {
        // Handle errors encountered during write
    });
}