Static & Private Class Fields

class APIClient {
  static #instance;
  #token;
  #baseURL;
  
  constructor(config) {
    if (APIClient.#instance) {
      return APIClient.#instance;
    }
    this.#baseURL = config.baseURL;
    this.#token = config.token;
    APIClient.#instance = this;
  }
  
  async #request(endpoint, options) {
    const response = await fetch(`${this.#baseURL}${endpoint}`, {
      ...options,
      headers: {
        'Authorization': `Bearer ${this.#token}`,
        ...options?.headers
      }
    });
    return response.json();
  }
}