Everything in node happens at the same time, which means, unless you're doing your processing inside your callbacks, you cannot guarantee asynchronous functions have completed yet.

async_iteration.js

/**
* Source: http://stackoverflow.com/questions/6287908/nodejs-express-fs-iterating-files-into-array-or-object-failing
*/

var fs = require('fs'),
    EventEmitter = require('events').EventEmitter,
    filesEmitter = new EventEmitter(),
    files = [];

// this event will be called when all files have been added to the files array
filesEmitter.on('files ready',function() {
  console.log(files);
});

// read all files from current directory
fs.readdir('.',function(err,files){
  if(err) throw err;
  files.forEach(function(file){
    files.push(file);
  });
  filesEmitter.emit('files ready'); // trigger files_ready event
});