Wrap your demo function:
(function(){
var demo = function(converter) {
return [
// Replace escaped @ symbols
{ type: 'lang', regex: '\\@', replace: 'red' }
];
};
// Client-side export
if (typeof window !== 'undefined' && window.Showdown && window.Showdown.extensions) { window.Showdown.extensions.demo = demo; }
// Server-side export
if (typeof module !== 'undefined') module.exports = demo;
}());