I have implemented a Store class which comes with a static factory method Store.create(), and I don’t want users to call the store’s constructor directly:

'use strict';

class Store {
  constructor (id) {
    this._id = id;
  }

  static create (id) {
    return new Store (id);
  }
}

module.exports = Store;

Ideally, I’d like to make constructor() private, but this concept does not exist in ES2015. So if I can’t hide the constructor, I want to forbid any direct calls. And tell the user what to do instead, if she does indeed mistakenly call new Store().

Check who’s calling

Here is the solution I have come up with:

'use strict';

const secret = {};

class Store {
  constructor (id, key) {
    if (key !== secret) {
      throw new Error ('Do not use new Store(): call Store.create() instead')
    }
    this._id = id;
  }

  static create (id) {
    return new Store (id, secret);
  }
}

module.exports = Store;

The constructor will only work if the caller passes in the expected secret. And since the object is only visible from inside the module, it cannot be passed in accidentally ({} is not equal to secret in the !== comparison).