code: function setStderrLevels (levels, debugStdout) {
      var defaultMsg = 'Cannot have non-string elements in stderrLevels Array';
      if (debugStdout) {
        if (levels) {
          //
          // Don't allow setting both debugStdout and stderrLevels together,
          // since this could cause behaviour a programmer might not expect.
          //
          throw new Error('Cannot set debugStdout and stderrLevels together');
        }
  
        return common.stringArrayToSet(['error'], defaultMsg);
      }
  
      if (!levels) {
        return common.stringArrayToSet(['error', 'debug'], defaultMsg);
      } else if (!(Array.isArray(levels))) {
        throw new Error('Cannot set stderrLevels to type other than Array');
      }
  
      return common.stringArrayToSet(levels, defaultMsg);
    }
docs: Convert stderrLevels into an Object for faster key-lookup times than an Array.  For backwards compatibility, stderrLevels defaults to ['error', 'debug'] or ['error'] depending on whether options.debugStdout is true.