Node.js asynchronous threads

Node.js has built-in mechanisms for creating threads and having them fire asynchronously. Using an example from http://book.mixu.net/node/ch7.html, we have the following:

//thread function - invoked for every number in items array 
function async(arg, callback) { 
  console.log('cube \''+arg+'\', and return 2 seconds later'); 
  setTimeout(function() { callback(arg * 3); }, 2000); 
} 
 
//function called once - after all threads complete 
function final() { console.log('Done', results); } 
 
//list of numbers to operate upon 
var items = [ 0, 1, 1, 2, 3, 5, 7, 11 ]; 
 
//results of each step 
var results = []; 
 
//loop the drives the whole process 
items.forEach(function(item) { 
  async(item, function(result){ 
    results.push(result); 
    if(results.length == items.length) { 
      final(); 
    } 
  }) 
}); 

This script creates an asynchronous function that operates on a number. For every number (item), we call upon the inline function, passing the number...