router问题

var http = require("http");
var router = require("./router.js");

//创建服务器
var server = http.createServer(function(req,res){
    if(req.url == "/"){
        router.showIndex(req,res);
    }else if(req.url.substr(0,9) == "/student/"){
        router.showStudent(req,res);
    }else{
        router.show404(req,res);
    }
});

server.listen(80,"127.0.0.1");

router.js

exports.showIndex = showIndex;
exports.showStudent = showStudent;
exports.show404 = show404;

function showIndex(req,res){
    res.writeHead(200,{"Content-Type":"text/html;charset=UTF8"});
    res.end("我是首页");
}

function showStudent(req,res){
    var id = req.url.substr(9,6);
    res.writeHead(200,{"Content-Type":"text/html;charset=UTF8"});
    res.end("我是学生页面" + id);
}

function show404(req,res){
    res.writeHead(404,{"Content-Type":"text/html;charset=UTF8"});
    res.end("404");
}