NodeJS中的exports语句

Question

I have the following code in server.js

var http = require('http');

function start() {
    function onRequest(request, response) {
        console.log('onrequest called');
        response.writeHead(200, { 'Content-type': 'text/plain' });
        response.write("Hello world!");
        response.end();
    }

    http.createServer(onRequest).listen(8888);
    console.log("Server started!");
}

exports.start = start;

And the following in index.js

var server = require('./server');
server.start();

What I'm not understanding is how the line exports.start = start; is working. Where is exports coming from? Why does index.js invoke the start method by server.start(); rather than exports.start() ? Isn't exports just a variable we put in the global namespace to make make a local var accessible to other modules by sticking it as a property on the global variable exports ?

Help!

Answer 1

Node wraps each module in it's own IIFE that provides arguments such as module , exports , __dirname , etc.

So when you write:

var http = require('http');

function start() {
    function onRequest(request, response) {
        console.log('onrequest called');
        response.writeHead(200, { 'Content-type': 'text/plain' });
        response.write("Hello world!");
        response.end();
    }

    http.createServer(onRequest).listen(8888);
    console.log("Server started!");
}

exports.start = start;

It's actually being wrapped in something like:

(function(module, exports, __dirname, ...) {
  var http = require('http');

  function start() {
      function onRequest(request, response) {
          console.log('onrequest called');
          response.writeHead(200, { 'Content-type': 'text/plain' });
          response.write("Hello world!");
          response.end();
      }

      http.createServer(onRequest).listen(8888);
      console.log("Server started!");
  }

  exports.start = start;
})(module, exports, __dirname, ...)

What I'm not understanding is how the line exports.start = start; is working. Where is exports coming from?

exports is an object just like any other JS object. You are attaching a reference to start on exports.start .

Why does index.js invoke the start method by server.start(); rather than exports.start()?

Good question. Since exports is just an object, exports.start won't reference anything unless you provide that reference by requiring a module.

You can, however, do this if your objective is to not have a local variable.

require('./server').start()