Singleton pattern คืออะไร?

SINGLETON → guarantee a class has only ONE instance, with global access to it:
  → the class controls its own instantiation (private constructor)
  → returns the SAME instance every time it's requested
→ for: things there should only be ONE of, accessed from many places

  {
   #instance;
   () {
     (!.#instance) {
      .#instance =  ();   
    }
     .#instance;             
  }
}