env.js

const { template } = require('./template')

function setEnv(env, paris) {

  for (const [name, value] of Object.entries(paris)) {

    let get
    if (typeof value === 'string') {

      const { dependencies, tplFunc } = template(value)
      if (dependencies) {
        get = () => {
          const values = {}
          for (const dep of dependencies) {
            if (dep === name) {
              const parentEnv = getParentEnv(env)
              const value = parentEnv ? parentEnv[dep] : undefined
              values[dep] = value === undefined ? `{{ ${dep} }}` : value
            } else {
              const value = env[dep]
              values[dep] = value === undefined ? `{{ ${dep} }}` : value
            }
          }
          return tplFunc(values)
        }
      } else {
        get = () => value
      }

    } else if (typeof value === 'function') {

      const func = value
      get = () => func(env)

    } else {

      get = () => value

    }

    Object.defineProperty(env, name, { get, enumerable: true, configurable: true })
  }
}

function showEnv(env) {
  const result = {}
  for (const name in env) {
    result[name] = env[name]
  }
  return result
}

function getParentEnv(env) {
  return Object.getPrototypeOf(env)
}

function createEnv(parentEnv = null, pairs = null) {

  const env = Object.create(parentEnv)
  if (pairs) setEnv(env, pairs)

  return env
}

module.exports = {
  createEnv,
  setEnv,
  showEnv,
  getParentEnv
}

template.js

const defaultRe = /\{\{\s*([a-zA-Z0-9_]+)\s*\}\}/

function template(tpl, re = defaultRe) {
  const matched = tpl.match(new RegExp(re, 'g'))
  if (!matched) {
    return { dependencies: null, tpl, tplFunc: () => tpl }
  } else {
    const tmp = {}
    for (const token of matched) {
      const [, name] = re.exec(token)
      tmp[name] = true
    }
    const dependencies = [...Object.keys(tmp)]
    const tplFunc = new Function('env', 'return' + JSON.stringify(tpl).replace(re, '" + env.$1 + "'))
    return { dependencies, tpl, tplFunc }
  }
}

module.exports = { template }