Static & Private Class Fields

class Form {
  #fields = new Map();
  #validators = new Map();
  
  addField(name, validator) {
    this.#fields.set(name, null);
    if (validator) {
      this.#validators.set(name, validator);
    }
  }
  
  setValue(name, value) {
    if (!this.#fields.has(name)) {
      throw new Error(`Field ${name} does not exist`);
    }
    const validator = this.#validators.get(name);
    if (validator && !validator(value)) {
      throw new Error(`Invalid value for ${name}`);
    }
    this.#fields.set(name, value);
  }
}