10. Extensible Class

(function extensibleClass() {
  let id = 0;
  return class Extensible {
    constructor() {
      this.id = id++;
    }

    extend(template) {
      Object.entries(template)
        .forEach(([key, value]) => {
          if (typeof value === 'function') {
            Object.getPrototypeOf(this)[key] = value;
          } else {
            this[key] = value;
          }
        });
    }
  };
}());