learnyounode-5.js

var fs = require('fs')
  , path = require('path')

fs.readdir(process.argv[2], function(err, list) {
  for (var k = 0; k < list.length; k++) {
    var name = list[k];
    if (path.extname(name) == '.' + process.argv[3]) console.log(name);
  }
});