如何为Node.js创建模块？

I have a simple code:

我有一个简单的代码：

var http = require("http");

var server = http.createServer(function(request, response) {
    response.writeHead(200, {"Content-Type" : "text/html"});
    response.write("Hello World");
    response.end();
});

server.listen(8000);

console.log("Server has started.");
var http =