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];
    }
  };
})();
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