Factory Functions & Singleton Pattern

What is problematic about this singleton implementation?

const Singleton = (() => {
  let instance;
  
  class Database {
    constructor() {
      if (instance) {
        return instance;
      }
      this.data = [];
      instance = this;
    }
    
    add(item) {
      this.data.push(item);
    }
  }
  
  return Database;
})();

It uses classes instead of factory functions
It doesn't actually create a singleton
The singleton check is in the constructor, which can be bypassed
This singleton implementation is problematic because: 1) The singleton check is in the constructor, which can be bypassed using Object.create(), 2) It doesn't prevent new instances when using inheritance, 3) It relies on a constructor returning a different object, which is unusual behavior, 4) If another developer sees the class, they might try to use it with 'new' without understanding the singleton logic, 5) It doesn't really follow the class paradigm's expected behavior, 6) A better approach would use static methods or a getInstance factory method for clarity.
It's not memory efficient

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