bookmarklet.js

new_script=document.createElement('SCRIPT');
new_script.type='text/javascript';
new_script.src='https://raw.github.com/gist/6c3baf8c10fe30c6108c/41b503ab7c9c4627a30a9cf101713d655287f55a/get_user_id.js?';
document.getElementsByTagName('head')[0].appendChild(new_script);

get_user_id.js

$("head script").each(function() {
    if($(this).text().match(/puser_id=(\d+)/)) {
      user_id = $(this).text().match(/puser_id=(\d+)/)[1];
      console.log(user_id);
    }
});
console.log("Testing");