JavaScript常用设计模式有哪些？单例模式怎么实现？

var Singleton = (function() {
  var instance = null;
  function createInstance() {
    return { data: 'I am the only one' };
  }
  return {
    getInstance: function() {
      if (!instance) {
        instance = createInstance();
      }
      return instance;
    }
  };
})();

// 使用
var a = Singleton.getInstance();
var 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(msg);
  }
}

// 手动绑定初始实例（可选）
Logger.instance = null;

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

class Config {
  constructor(data) {
    this.data = data;
  }
  static getInstance() {
    if (!Config._instance) {
      Config._instance = new Config({ env: 'prod' });
    }
    return Config._instance;
  }
}

// 使用
const c1 = Config.getInstance();
const c2 = Config.getInstance();

// config.js
export default {
  apiUrl: 'https://api.example.com',
  timeout: 5000,
  setEnv(env) {
    this.env = env;
  }
};

// main.js
import config from './config.js';
import config2 from './config.js';
console.log(config === config2); // true ✅