How to define private constructors in javascript?

javascript

Solution

Is there a solution which will allow me to say `x instanceof Score`?

Yes. Conceptually, @user2864740 is right, but for `instanceof` to work we need to expose (`return`) a function instead of a plain object. If that function has the same `.prototype` as our internal, private constructor, the `instanceof` operator does what is expected:

var Score  = (function () {

  // the module API
  function PublicScore() {
    throw new Error('The constructor is private, please use Score.makeNewScore.');
  }

  // The private constructor
  var Score = function (score, hasPassed) {
      this.score = score;
      this.hasPassed = hasPassed;
  };

  // Now use either
  Score.prototype = PublicScore.prototype; // to make .constructor == PublicScore,
  PublicScore.prototype = Score.prototype; // to leak the hidden constructor
  PublicScore.prototype = Score.prototype = {…} // to inherit .constructor == Object, or
  PublicScore.prototype = Score.prototype = {constructor:null,…} // for total confusion :-)

  // The preferred smart constructor
  PublicScore.mkNewScore = function (score) {
      return new Score(score, score >= 33);
  };

  return PublicScore;
}());

> Score.mkNewScore(50) instanceof Score
true
> new Score
Error (…)

Problem

I have defined pure objects in JS which expose certain static methods which should be used to construct them instead of the constructor. How can I make a constructor for my class private in Javascript? ``` var Score = (function () { // The private constructor var Score = function (score, hasPassed) { this.score = score; this.hasPassed = hasPassed; }; // The preferred smart constructor Score.mkNewScore = function (score) { return new Score(score, score >= 33); }; return Score; })(); ``` Update: The solution should still allow me to test for `x instanceof Score`. Otherwise, the solution by @user2864740 of exposing only the static constructor works.

Original source