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:

 1'use strict';
 2
 3class Store {
 4  constructor (id) {
 5    this._id = id;
 6  }
 7
 8  static create (id) {
 9    return new Store (id);
10  }
11}
12
13module.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:

 1'use strict';
 2
 3const secret = {};
 4
 5class Store {
 6  constructor (id, key) {
 7    if (key !== secret) {
 8      throw new Error ('Do not use new Store(): call Store.create() instead')
 9    }
10    this._id = id;
11  }
12
13  static create (id) {
14    return new Store (id, secret);
15  }
16}
17
18module.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).