async_function_with_callback.js

var find_out_meaning_of_life = function (is_it, callback) {
  var meaning_of_life = 42;
  try {
    setTimeout(function () {
      callback(null, (is_it === meaning_of_life))
    }, 420);
  } catch(err) {
    callback(err, 'FAIL!');
  }
};

var philosopher_say_it_loud = function (err, result) {
  if (err) {
    console.log(err);
  } else {
    if (result)
      console.log('Heureka!', result);
    else
      console.log('Darn!');
  }
};

find_out_meaning_of_life(41, philosopher_say_it_loud);
find_out_meaning_of_life(42, philosopher_say_it_loud);

async_function_with_events.js

// With events
var events = require('events');

function Philosopher() {
  this.meaning_of_life = 42;
  events.EventEmitter.call(this);
}

Philosopher.super_ = events.EventEmitter;
Philosopher.prototype = Object.create(events.EventEmitter.prototype, {
    constructor: {
        value: Philosopher,
        enumerable: false
    }
});


Philosopher.prototype.find_out_meaning_of_life = function(is_it) {
  var self = this;

  setTimeout(function () {
    self.emit('result', is_it === self.meaning_of_life );
  }, 420);

  return self;
}

var aristoteles = new Philosopher();

aristoteles.on('result', function(result) {
  if (result) {
    console.log('Heureka!');
  } else {
    console.log('Darn...');
  }
})

aristoteles.find_out_meaning_of_life(32);
aristoteles.find_out_meaning_of_life(42);

async_function_with_promises.js

// with promise
// https://github.com/cujojs/when
var when = require('when');

var find_out_meaning_of_life = function (is_it) {
  var deferred = when.defer();
  var meaning_of_life = 42;

  setTimeout(function () {
      deferred.resolve((is_it === meaning_of_life));
  }, 420);

  return deferred.promise;
};

var philosopher_say_it_loud = function (result) {
  if (result)
    console.log('Heureka!');
  else
    console.log('Darn!');
};

find_out_meaning_of_life(41).then(philosopher_say_it_loud);
find_out_meaning_of_life(42).then(philosopher_say_it_loud);