A short demonstration on asynchronous processing with node.

asyncnode.js

/*
 * A short demonstration on asynchronous processing with node.
 */

// Asynchronous adding method
var add = function(a, b, timeout, callback) {
  setTimeout(function() {
    callback(a + b);
  }, timeout * 1000); // Sleeps for <timeout> miliseconds.
};

// Asynchronous subtracting method
var subtract = function(a, b, timeout, callback) {
  setTimeout(function() {
    callback(a - b);
  }, timeout * 1000); // Sleeps for <timout> miliseconds.
};

addThenSubtract = function(a, b, c, callback) {
  // Add a and b
  console.log('start adding');
  add(a, b, 1, function(res) {
    console.log("result for adding: " + res);
  });
  console.log('start subtracting');
  // Subtract b and c
  subtract(b, c, 1, function(res) {
    console.log("result for subtractor: " + res);
  });
  console.log('next process');
};

addThenSubtract(1,3,2);
/*
 * What you normally expect from synchronous process:
 * start adding
 * result for adding: 4
 * start subtracting
 * result for subtracting: 1
 * next process
 *
 * What actually happens:
 * start adding
 * start subtracting
 * next process                 // You will notice that it didn't wait
 * result for adding: 4         // for the long process to finsh. Instead,
 * result for subtracting: 1    // it went on with the next line of code).
 */