✨ 12 Factor-compliant config loader script for environment variable

GitHub stars

This article contains some code template that would elaborate you from the dull routine of adding process.env || at the beginning of each Node.js configuration file. Following the 12 Factor's config methodology, configs are stored in the environment variable, while also be able to read from files for default setting/ bootstrapping, etc.

Motivation

The motivation of the project are from the ES6 tutorial and SmartVIS Back-End project that I've been doing. I'm excited by the idea of using the ES6's Proxy, Reflect and Symbol feature to introduce an automatic environmental variable loader, without hampering how the rest of code is retrieving configs from file.

// src/app.js
const config = require("../config");

So… What it's like?

We want to keep it this way, and with 50 lines of code, the explicitly annoying process.env is now exempted from config implementation.

// FROM: config.js (or config/index.js)
module.exports = {
  env: {
    PORT: process.env.PORT || 3000
  },
  database: {
    HOST: process.env.DATABASE_HOST || "mysql",
    PORT: process.env.DATABASE_PORT || "3306",
    USER: process.env.DATABASE_USER || "example",
    PASSWORD: process.env.DATABASE_PASSWORD || "example",
    DATABASE: process.env.DATABASE_DATABASE || "example"
  }
};

// TO: config/default.js
module.exports = {
  env: {
    PORT: 3000
  },
  database: {
    HOST: "mysql",
    PORT: "3306",
    USER: "example",
    PASSWORD: "example",
    DATABASE: "example"
  }
};

Intuition and Installation

But how can we possibly achieve this? The answer is the magical config/index.js that I'm going to introduce to you. The intuition is quite clear and straight forward: whenever a client try to get a certain config, we first search for process.env and otherwise return the default value (process.env.KEY || 'default_value'). An extra key is required for configs of process.env.SERVICE_FIELD in order to store the previously called service. ES6's Proxy, Reflect and Symbol feature are required as to intercept the getter. You may check for implementation for more detail.

To get the power of no-more-explict-process-env, just download the config/index.js and organized it like the templates, and enjoy!

Show me the code

/* eslint-disable no-process-env */
"use strict";

const config = require("./default");

const KEY = Symbol();

function isObject(value) {
  return value && typeof value === "object" && value.constructor === Object;
}

function ProxyFactory(obj) {
  /**
   * Handler with getter to return env/value/Proxy<-object when inspecting configs
   *
   * @type {{get}}
   */
  const configHandler = {
    get: (target, key, receiver) => {
      // Concat to the matched name required for env
      // e.g. config.oss.sts.ROLE_ARN -> OSS_STS_ROLE_ARN
      const envName = [Reflect.get(target, KEY, receiver), key]
        .filter(Boolean)
        .map(_ => _.toString().toUpperCase())
        .join("_");
      // Return the environment variable if there is one
      const p = process.env[envName];
      if (p) {
        return p;
      }
      // Get original value/object
      const o = Reflect.get(target, key, receiver);
      // Return the value/undefined
      if (!isObject(o)) {
        return o;
      }
      // Return ProxyFactory(obj) with new object appended with a Symbol key to store envName
      return ProxyFactory(
        {
          ...o,
          [KEY]: envName
        },
        configHandler
      );
    }
  };
  return new Proxy(obj, configHandler);
}

/**
 * Loader for environmental variable configs named after <SERVICE_FIELD> if set else config
 *
 * @type {Proxy}
 */
module.exports = ProxyFactory(config);

TOC