How to define private constructors in javascript?

One can use a variable (initializing) inside a closure which can throw an error if the constructor was called directly instead of via a class method:

var Score = (function () {
  var initializing = false;

  var Score = function (score, hasPassed) {
      if (!initializing) {
         throw new Error('The constructor is private, please use mkNewScore.');
      }

      initializing = false;
      this.score = score;
      this.hasPassed = hasPassed;
  };

  Score.mkNewScore = function (score) {
      intializing = true;
      return new Score(score, score >= 33);
  };

  return Score;
})();