example.js

var http = require('http'),
    url = require('url');

var server = http.createServer(onRequest)
    
server.listen(8888);

function onRequest(request, response) {
    var parsedUrl = url.parse(request.url, true),
        urlToExpand = parsedUrl.query.url;
    
    expand(urlToExpand, writeResponse);
    
    function writeResponse(error, newUrl) {
        if (error) {
            response.writeHead(500, { 'Content-Type': 'text/plain'});
            return response.end(error.message);
        }
        response.writeHead(200, { 'Content-Type': 'text/plain'});
        response.end(newUrl);
    }
}

function expand(urlToParse, callback) {
    var short = url.parse(urlToParse);
    var options = {
        host: short.hostname,
        port: 80,
        path: short.pathname
    };
    var clientRequest = http.get(options, extractRealURL);
    
    clientRequest.on("error", forwardError);
    
    function extractRealURL(res) {
        callback(null, res.headers.location);    
    }
    
    function forwardError(error) {
        callback(err);    
    }
}

This code fails if you do not enter a url parameter. You need to check if parsedUrl.query.url is defined before continuing with call to expand(urlToExpand, writeResponse);