Object with root-level metadata

object-level-metadata.js

/**
 * Getters and setters are not enumerable and, obviously, neither are private fields.
 */
class MetadataObject extends Object {
  #metadata = undefined;
  
  get metadata() { return this.#metadata; }
  set metadata(value) { this.#metadata; }
}

var obj = new MetadataObject();
obj.a = 1;
obj.b = 2;
obj.metadata = { test: 123 };

console.log(obj); // { a: 1, b: 2 }
console.log(obj.metadata); // { test: 123 }
console.log(JSON.stringify(obj)); // '{"a":1,"b":2}'

/**
 * Simpler implementation using Map
 */
class MetadataObject extends Object {
  #metadata = new Map();
  
  get metadata() { return this.#metadata; }
}

var obj = new MetadataObject();
obj.a = 1;
obj.b = 2;
obj.metadata.set('test', 123);

console.log(obj); // { a: 1, b: 2 }
console.log(obj.metadata.get('test')); // 123
console.log(JSON.stringify(obj)); // '{"a":1,"b":2}'