Access local page variables inside Chrome/Firefox userscript

To access a page's local variables from a userscript, use a callback:

// ==UserScript==
// @name  Example Script
// @match https://stackoverflow.com/*
// ==/UserScript==

var callback = function() {
    alert('The page is running jQuery version: ' + $.fn.jquery);
};

window.location = 'javascript:(' + callback.toString() + ')();void(0)';

In your callback, be sure to only use multiline comments if needed ("/* comment */") as single line comments ("//"), will prevent the callback from executing.

kw: jQuery, unsafewindow, greasemonkey, chrome, firefox