Factory Functions & Singleton Pattern

What pattern does this code implement?

const Registry = (() => {
  const instances = {};
  
  return {
    register(key, instance) {
      if (!instances[key]) {
        instances[key] = instance;
      }
      return instances[key];
    },
    get(key) {
      return instances[key] || null;
    },
    remove(key) {
      delete instances[key];
    }
  };
})();

Simple singleton
Multiton pattern
This code implements the multiton pattern: 1) It's an extension of the singleton pattern that manages multiple named instances, 2) Each key has at most one associated instance, 3) It provides a registry of singletons identified by keys, 4) It offers controlled access to the registered instances, 5) This pattern is useful for managing a limited set of instances, like connection pools or caches, 6) It combines the instance management of singletons with the flexibility of multiple controlled instances.
Factory pattern
Builder pattern

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