config-loader.js

import * as requireYaml from "require-yml";
import * as set from "lodash.set";
import * as merge from "deepmerge";
import { load as parseYaml } from "js-yaml";
import { readFileSync } from "fs";

type StringMap = { [key: string]: string };

let config: any, configRootPath: string;
_resetConfig();

export const configLoader = (process = global.process) => {
  if (config) return config;

  const { name, version } = JSON.parse(
    readFileSync(process.cwd() + "/package.json").toString()
  );
  const { fromEnv = {}, ...defaults } = loadFile(configRootPath);
  const env = parseEnvVars(process.env, fromEnv);
  const switches = parseCliSwitches(process.argv);

  return (config = merge.all([
    { package: { name, version } },
    defaults,
    env,
    switches,
  ]));
};

function loadFile(configRootPath) {
  const conf = requireYaml(configRootPath);
  if (!conf || !Object.keys(conf).length) {
    const cwd = process.cwd();
    const msg = `no config found at: ${configRootPath} (cwd: ${cwd})`;
    throw Object.assign(new ConfigError(msg), { cwd });
  }
  return conf;
}

/**
 * @param envParams
 *  any process.env equivalent
 * @param fromEnv
 *  an object who's keys are names of env-vars,
 *  and its values are paths on the config object to override in case this env-variable is found.
 * @returns config object
 */
export const parseEnvVars = (envParams: StringMap, fromEnv: StringMap) =>
  Object.entries(fromEnv).reduce(
    (config, [paramName, configPath]) =>
      envParams.hasOwnProperty(paramName)
        ? set(config, configPath, parseYaml(envParams[paramName]))
        : config,
    {}
  );

class ConfigError extends Error { pwd: string; }
class CliSwitchError extends Error {}

/**
 * @param argv any process.argv equivalent
 * @returns a config object
 */
export const parseCliSwitches = (argv: Array<string>) => {
  let current = argv[2];
  const rest = argv.slice(3);
  const config = {};

  rest.concat([null]).forEach((next) => {
    if (next === null)
      if (current == null) return;
      else next = "true";

    //current is an implicit boolean
    if (next.startsWith("-")) {
      set(config, current, true);
      current = next.replace(/^-{1,2}/, "");
      return;
    }

    //next is value for current
    if (current === null)
      throw new CliSwitchError("found a value without a preceding switch");

    set(config, current.replace(/^-{1,2}/, ""), parseYaml(next));
    current = null;
  });

  return config;
};

/**
  for unit tests
 */
export const resetConfig = _resetConfig;

function _resetConfig(crp = "./config") {
  configRootPath = crp;
  config = null;
}