為什么會出現(xiàn)這個 undefined呢，為什么端口號沒有定義

求告知

下面是js的文件

var express = require('express');
var config = require('./config/index');

var port = process.env.PORT || config.build.port;

var app = express();

var router = express.Router();

router.get('/', function (req, res, next) {

req.url = '/index.html';
next();

});

app.use(router);

var appData = require('./data.json');
var seller = appData.seller;
var goods = appData.goods;
var ratings = appData.ratings;

var apiRoutes = express.Router();

apiRoutes.get('/seller', function (req, res) {

res.json({
    errno: 0,
    data: seller
});

});

apiRoutes.get('/goods', function (req, res) {

res.json({
    errno: 0,
    data: goods
});

});

apiRoutes.get('/ratings', function (req, res) {

res.json({
    errno: 0,
    data: ratings
});

});

app.use('/api', apiRoutes);

app.use(express.static('./dist'));

module.exports = app.listen(port, function (err) {

if (err) {
    console.log(err);
    return
}
console.log('Listening at http://localhost:' + port + '\n')

});