Experiment for testing the "Concurrency Model and Event Loop" of JavaScript.

ConcurrencyModelOfJavaScript.js

/**
 * This is an experiment for testing the "Concurrency Model and Event Loop" of
 * JavaScript.
 *
 * It's designed that an asynchronous function with a given callback is called;
 * And subsequently a synchronous function which is designated to take far more
 * time than the asynchronous one will take is called.
 *
 * Question: what is the response order of async-fun/sync-fun/callback?
 *
 * Answer: JavaScript runtime contains a message queue which stores a list of
 * messages to be processed and their associated callback functions. The main
 * thread is taken as a message queued with toppest executing priority. And
 * JavaScript runtime process each message completely before any other message
 * is processed. This differs from C, for instance, where if a function runs in
 * a thread, it can be stopped at any point to run some other code in another
 * thread.
 * In the example, even the asynchronous callback takes far less time to finish
 * than the synchronous function will take. The async-function is called first;
 * and the sync-function is called secondly; then the callback is called lastly.
 *
 * Usage
 * -----
 * node ConcurrencyModelOfJavaScript.js
 *
 * Reference
 * ---------
 * - http://blog.carbonfive.com/2013/10/27/the-javascript-event-loop-explained
 * - https://developer.mozilla.org/en-US/docs/Web/JavaScript/EventLoop
 */

var http = require('http');
var countDown = Math.pow(2, 30);

function syncWait(loop) {
  var counter = loop;

  while (counter-- >= 0);
}

console.time('Asynchronous function called (with callback).');
http.get({host: 'www.google.com'}, function(res) {
  console.log('Asynchronous callback called. res.statusCode = %s', res.statusCode);
});
console.timeEnd('Asynchronous function called (with callback).');

console.time('Synchronous function called.');
syncWait(countDown);
console.timeEnd('Synchronous function called.');

// output example:
// Asynchronous function called (with callback).: 5ms
// Synchronous function called.: 1260ms
// Asynchronous callback called. res.statusCode = 302