请求和响应来自哪里？

Question

On line 2, there are 'request' and 'response' parameters to be received, yet on line 6, in the createServer call, I pass it the request handler function, with no arguments. Where do these two specific parameters come from if I'm not passing them on invocation?

var http = require("http");
function requestHandler(request, response) {
  console.log();
  response.end();
}
var server = http.createServer(requestHandler);
server.listen(3000);

Answer 1

// importing the http module
var http = require("http"); 

// defining what happens when a request hit the server (a.k.a callback function)
function requestHandler(request, response) {
  console.log();
  response.end();
}

// creating a server and linking the previously defined request handler.
var server = http.createServer(requestHandler);
server.listen(3000);

Since you have already defined what this handler takes in as params and what it does, you just need to provide the name of the function to link it to your newly created server.

In other words you are passing the function itself as a parameter to the createServer function. Functions are first class in JavaScript.

The params that you name in the request handler helps you read from the request and write to the response.

Answer 2

The request and response parameters are coming from the script that will actually call your request handler.

The node server, when listening on the 3000 port, will catch requests, do some internal management on them (I assume, I don't exactly know what's going on down there), create a response object, and call your handler in a context that will allow it to return content.

---

Here is a very simple unrelated synchronous exemple. When defining callback , we just define a function that will log whatever it is passed. That function is then passed to usesCallback , which will use it to accomplish the log.

function callback(value) {
    console.log(value);
}
function getValue() {
    return 42;
}
function usesCallback(callback) {
    var value = getValue();
    callback(value);
}

usesCallback(callback);
// logs 42 to the console