learnyounode make it modular

extmodule.js

var fs = require('fs');  
var path =  require('path')
module.exports = function(dir,extension,cbf){
  fs.readdir(dir,function(err,files){
    if(err){
      cbf(err,null);
    }
    else{
      cbf(null,files.filter(function(val){
        if(path.extname(val)=='.'+extension){
          return val;
        }
      }));
    }});
}

program.js

var lib = require('./extmodule.js');
var dir = process.argv[2];
var ext = process.argv[3];
lib(dir,ext,function(err,files){
  if(err)
    console.log(err);
  else{
    files.forEach(function(val){
      console.log(val);
    });
  }
});