代码：https://github.com/fengchunjian/nodejs_examples/tree/master/routerv3

//vim models/optfile.js
var fs = require('fs')
module.exports = {
    writefile : function(path, data, recall) {
        fs.writeFile(path, data, function(err) {
            if (err) {
                throw err;
            }
            console.log("异步写文件完成");
            recall("异步写文件完成");
        });
    },
    writefileSync : function(path, data, res) {
        fs.writeFileSync(path, data);
        console.log("同步写文件完成");
        res.write("同步写文件完成");
    },
    readfile : function(path, recall) {
        fs.readFile(path, function(err, data) {
            if (err) {
                console.log(err);
            } else {
                recall(data);
            }
        });
        console.log("异步方法执行完毕");
    },
    readfileSync : function(path, res) {
        var data = fs.readFileSync(path, "utf-8");
        console.log(data);
        console.log("同步方法执行完毕");
        res.write(data);
    }
}

//vim models/router.js
var optfile = require("./optfile");
module.exports = {
    writefile : function(req, res) {
        function recall(data) {
            res.write(data);
            res.end();
        }
        optfile.writefile("./file.txt", "异步文件写入", recall);
    },
    writefileSync : function(req, res) {
        optfile.writefileSync("./sync.txt", "同步文件写入", res);
        res.end();
    },
    login : function(req, res) {
        function recall(data) {
            res.write(data);
            res.end();
        }
        optfile.readfile("./views/login.html", recall);
    },
    zhuce : function(req, res) {
        function recall(data) {
            res.write(data);
            res.end();
        }
        optfile.readfile("./views/zhuce.html", recall);
    }
}

//vim routercall.js
var http = require('http');
var url = require('url');
var router = require('./models/router');
http.createServer(function (request, response) {
    var pathname = url.parse(request.url).pathname;
    pathname = pathname.replace(/\//, '');
    router[pathname](request, response);
    console.log("主程序执行完毕");
}).listen(8000);
console.log('Server running at http://127.0.0.1:8000/');

node routercall.js
Server running at http://127.0.0.1:8000/
同步写文件完成
主程序执行完毕
主程序执行完毕
异步写文件完成

curl http://localhost:8000/writefileSync
同步写文件完成
curl http://localhost:8000/writefile
异步写文件完成

cat sync.txt
同步文件写入
cat file.txt
异步文件写入

参考文档

nodejs6_写文件(n6_write)
http://www.yuankuwang.com/web/index.php?r=respool/resview&rpid=38
node.js教程6_写文件
http://edu.51cto.com/center/course/lesson/index?id=124530