code: function (value) {
      const IllegalValue = this.$err.IllegalValue;
      if (typeof value !== 'string') {
        return value;
      }
      const result = value.replace(ENVVAR_PATTERN, function () {
        const varname = arguments[1];
        if (process.env[varname] === undefined) {
          throw new IllegalValue(`Environment variable ${varname} was referenced but not set`);
        }
        return process.env[varname];
      });
  
      return result;
    }
docs: Interpolates config values with the values of the environment variables of the process.
  
  @private
  @param {any} value - The value specified for a parameter; possibly an environment variable.