index.js

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

var config = module.exports = function(opts) {
	opts = opts || {};

	// path.resolve() will take absolute paths and CWD into account
	var dir = path.resolve(opts.dir || 'config');
	var env = opts.env || process.env.NODE_ENV || 'development';
	// You can specify the extension on opts.ext to improve performance
	var ext = 'ext' in opts ? opts.ext : extension(dir, env);
	var file = path.join(dir, env) + (ext ? '.'+ext : '');
	// You can pass an option named as the extension with a function that parses the file
	// else handled by native require() (directory(''), js, json and others) 
	var data = ext in opts ? opts[ext](fs.readFileSync(file, 'utf8')) : require(file);
	copy(config, data);

	return config;
};

function extension(dir, env) {
	var file = fs.readdirSync(dir).sort().filter(function(filename) {
		// Directory
		return filename === env ||
			// File
			filename.split('.').slice(0, -1).join('.') === env;
	})[0];

	if (!file) {
		throw new Error('No file found for environment '+env);
	}
	return file.slice(env.length+1);
}

function copy(dest, src) {
	for (var key in src) {
		var d = dest[key];
		var s = src[key];
		if (object(d) && object(s)) {
			copy(d, s);
		} else {
			dest[key] = s;
		}
	}
}

function object(obj) {
	return Object.prototype.toString.call(obj) === '[object Object]';
}