求教。。。这个undefined is not a function是为啥？

var app = require(’…/app’); var debug = require(‘debug’)(‘blog:server’); var http = require(‘http’);

// Get port from environment and store in Express.

var port = normalizePort(process.env.PORT || ‘3000’); app.set(‘port’, port);//就是说我这部分有问题

var server = http.createServer(app);

server.listen(port); server.on(‘error’, onError); server.on(‘listening’, onListening);