client.makeCopyReference('/d.txt', function (error, copyref) {
if (error) {
    return handleError(error); // Handle the error.
}

client.copy(copyref, '/test_folder/kamret.txt', function (error, stat) {
    if (error) {
        return handleError(error);  // An error is thrown here!
    }

    $('#resultBox').html(stat.name + " is succesfully copied!");
});