.env

DB_URL=mongo://...

app.js

/**
 * Module deps
 */
var config = require("config");

// Get the DB
var DB_URL = config("DB_URL", "You need to set DB_URL for the mongo instance");

// Throws an exception
var REDIS_URL = config("REDIS_URL", "You need to set REDIS_URL for the redis instance");

config.js

/**
 * Get the config
 */
 
var defaults = {};

exports = module.exports = function(name, message) {
  if(process.env[name]) return process.env[name];
  if(defaults[name]) return defaults[name];
  throw new Error(message);
}

exports.default = function(key, value) {
  defaults[key] = value;
}