Replace the attribute value using cheerio

The following code is used to replace all the <img> tags src value. But the following code does not modify the original document. $.html prints the original document and not the modified one.

    $ = cheerio.load(data);
    $("img").each(function() {
        var old_src=$(this).attr("src");
        var new_src = "/my_cached_image?url=" + encodeURIComponent(old_src);        
        $(this).prop("src", new_src);
    });
    modified_data = $.html();