Config reader based on node config module

config.js

var inherit = require('inherit'),
    extend = require('extend');

module.exports = inherit({
    __constructor: function(opts, overrides) {
        // Remove config module from cache so it could be recreated.
        // It's a limitation of the config module.
        delete require.cache[require.resolve('config')];

        resetEnvVars();
        setEnvVars(opts);
        var conf = require('config');
        resetEnvVars();

        if (overrides) {
            conf = extend(true, conf, overrides);
        }
        return conf;
    }
});

var envVars = [
        'NODE_ENV',
        'NODE_CONFIG_DIR',
        'HOSTNAME',
        'HOST',
        'NODE_APP_INSTANCE',
        'ALLOW_CONFIG_MUTATIONS',
        'NODE_CONFIG_STRICT_MODE',
        'SUPPRESS_NO_CONFIG_WARNING'
    ],
    savedVars = {};

function saveEnvVars() {
    envVars.forEach(function(v) {
        if (typeof process.env[v] !== 'undefined') {
            savedVars[v] = process.env[v];
        }
    });
}

function resetEnvVars() {
    envVars.forEach(function(v) {
        if (savedVars[v]) {
            process.env[v] = savedVars[v];
        } else {
            delete process.env[v];
        }
    });
}

var optsEnvVarsMap = {
    env: 'NODE_ENV',
    configDir: 'NODE_CONFIG_DIR',
    host: 'HOSTNAME',
    instance: 'NODE_APP_INSTANCE'
};

function setEnvVars(opts) {
    Object.keys(optsEnvVarsMap)
        .forEach(function(key) {
            if (typeof opts[key] !== 'undefined') {
                process.env[optsEnvVarsMap[opts[key]]] = opts[key];
            }
        });
}

// Save config module related environment variables for later use
saveEnvVars();