Node.js _http_server.js的服务器功能的第一行混乱

Question

I'm reading node.js' http modules' source code. In _http_server.js it has a function

function Server(requestListener) {
  if (!(this instanceof Server)) return new Server(requestListener);
  net.Server.call(this, { allowHalfOpen: true });

  if (requestListener) {
    this.on('request', requestListener);
  }
  ...
}

I can't understand the first line if (!(this instanceof Server)) return new Server(requestListener); .

In other files for example the function http.createServer , this function is called as return new Server(requestListener) . Because the object that new create is never an instance of Server function. Won't it be an infinite recursive loop?

---

Answers below are great. But here is a little complement to my confusion:

By calling new Foo() , a new object will be created. Then object 's [[prototype]] will be linked to Foo 's .prototype . Only after that this will be bound to object and Foo will be executed.

Since instanceof check whether Foo 's prototype is in object 's prototype chain, if Server is called by new then the newly created object will be Server 's instance.

Answer 1

This is a common idiom to allow instantiation of a class without having to use new . In other words, it allows both of these:

let instance = new Server();
let instance = Server();

That first line is meant to catch the second method of instantiating (which is just a regular function call). In that case, this doesn't refer to the new instance, so this instanceof Server will be false.

When that happens, an instance is creating using new Server() . The same function will be called again, but because of using new the this variable will now point to the newly created instance and the check bypassed (so it doesn't create a recursive call).

In other files for example the function http.createServer , this function is called as return new Server(requestListener) . Because the object that new create is never an instance of Server function.

It's an instance of the Server class .

Answer 2

When you create an element with new , the object created is the instance of the function referenced as new , so when http.createServer calls return new Server(requestListener) then the new element is an instance of Server .

This way is impossible to have a loop because when is not an instance, it creates a new instance, being this way:

• I'm called, let's check if I'm an instance of me
• Nope, create a new instance of me
• I'm called, let's check if I'm an instance of me
• Yep, I'm an instance of me. Move on

This is done to create a new instance of Server without the need to call new .

var serverinstance = Server(rqlistener);
var serverinstance2 = new Server(rqlistener);

Both are an instance of Server , because of the inner code of the Server function.

Under my opinion, I don't like these kind of hacks btw.