code: function required (key) {
    if (missing(key)) {
      throw new ReferenceError(format(REQUIRED_ERROR, key))
    } else {
      return get(key)
    }
  }
docs: Throw a `ReferenceError` if given key is missing from `process.env` and return given key's value from `process.env` otherwise.
  aliases: assert
  
  @param {String} key
  @return {String}
  @api public
  @throws {ReferenceError}