const Singleton = (function() {
  let instance = null;
  function createInstance() {
    return { data: 'shared config', getTime: () => new Date() };
  }
  return {
    getInstance() {
      if (!instance) {
        instance = createInstance();
      }
      return instance;
    }
  };
})();

// 使用
const a = Singleton.getInstance();
const b = Singleton.getInstance();
console.log(a === b); // true

class Logger {
  constructor() {
    if (Logger.instance) {
      return Logger.instance;
    }
    this.logs = [];
    Logger.instance = this;
  }
  log(msg) {
    this.logs.push(`[${new Date().toISOString()}] ${msg}`);
  }
}

// 手动初始化一次（或延迟到首次调用）
Logger.instance = null;

// 使用
const logger1 = new Logger();
const logger2 = new Logger();
console.log(logger1 === logger2); // true

// logger.js
export const logger = {
  logs: [],
  log(msg) {
    this.logs.push(`[${new Date().toISOString()}] ${msg}`);
  }
};

// 在任何地方 import，拿到的都是同一个对象
// import { logger } from './logger.js';