如果你想使用类:

class Singleton {
  constructor(name, age) {
    this.name = name;
    this.age = age;
    if(this.constructor.instance)
      return this.constructor.instance;
    this.constructor.instance = this;
  }
}
let x = new Singleton('s', 1);
let y = new Singleton('k', 2);

以上的输出将是:

console.log(x.name, x.age, y.name, y.age) // s 1 s 1

另一种使用函数编写Singleton的方法

function AnotherSingleton (name,age) {
  this.name = name;
  this.age = age;
  if(this.constructor.instance)
    return this.constructor.instance;
  this.constructor.instance = this;
}

let a = new AnotherSingleton('s', 1);
let b = new AnotherSingleton('k', 2);

以上的输出将是:

console.log(a.name, a.age, b.name, b.age) // s 1 s 1

最明确的答案应该是Addy Osmani所著的《学习JavaScript设计模式》一书。

var mySingleton = (function () { // Instance stores a reference to the singleton var instance; function init() { // Singleton // Private methods and variables function privateMethod(){ console.log( "I am private" ); } var privateVariable = "I'm also private"; var privateRandomNumber = Math.random(); return { // Public methods and variables publicMethod: function () { console.log( "The public can see me!" ); }, publicProperty: "I am also public", getRandomNumber: function() { return privateRandomNumber; } }; }; return { // Get the singleton instance if one exists // or create one if it doesn't getInstance: function () { if ( !instance ) { instance = init(); } return instance; } }; })();

简短的回答:

由于JavaScript的非阻塞特性，JavaScript中的单例在使用时非常难看。全局变量将在整个应用程序中为您提供一个实例，而不需要所有这些回调，模块模式将内部隐藏在接口后面。参见Christian C. Salvadó的回答。

但是，既然你想要一个单胞胎……

var singleton = function(initializer) {

  var state = 'initial';
  var instance;
  var queue = [];

  var instanceReady = function(createdInstance) {
    state = 'ready';
    instance = createdInstance;
    while (callback = queue.shift()) {
      callback(instance);
    }
  };

  return function(callback) {
    if (state === 'initial') {
      state = 'waiting';
      queue.push(callback);
      initializer(instanceReady);
    } else if (state === 'waiting') {
      queue.push(callback);
    } else {
      callback(instance);
    }
  };

};

用法:

var singletonInitializer = function(instanceReady) {
  var preparedObject = {property: 'value'};
  // Calling instanceReady notifies the singleton that the instance is ready to use
  instanceReady(preparedObject);
}
var s = singleton(singletonInitializer);

// Get the instance and use it
s(function(instance) {
  instance.doSomething();
});

解释:

单例在整个应用程序中提供了不止一个实例:它们的初始化延迟到第一次使用。当您处理初始化开销很大的对象时，这确实是一个大问题。昂贵通常意味着I/O，在JavaScript中I/O总是意味着回调。

不要相信那些给出instance = singleton.getInstance()这样接口的答案，它们都没有抓住重点。

如果它们没有在实例就绪时运行回调，那么当初始化器执行I/O时，它们将不起作用。

是的，回调看起来总是比函数调用更丑，函数调用会立即返回一个对象实例。但是同样:当你执行I/O时，回调是必须的。如果你不想做任何I/O，那么在程序启动时实例化就足够便宜了。

例1，简单的初始化器:

var simpleInitializer = function(instanceReady) {
  console.log("Initializer started");
  instanceReady({property: "initial value"});
}

var simple = singleton(simpleInitializer);

console.log("Tests started. Singleton instance should not be initalized yet.");

simple(function(inst) {
  console.log("Access 1");
  console.log("Current property value: " + inst.property);
  console.log("Let's reassign this property");
  inst.property = "new value";
});
simple(function(inst) {
  console.log("Access 2");
  console.log("Current property value: " + inst.property);
});

例2，初始化I/O:

在这个例子中，setTimeout模拟了一些昂贵的I/O操作。这说明了为什么JavaScript中的单例真的需要回调。

var heavyInitializer = function(instanceReady) {
  console.log("Initializer started");
  var onTimeout = function() {
    console.log("Initializer did his heavy work");
    instanceReady({property: "initial value"});
  };
  setTimeout(onTimeout, 500);
};

var heavy = singleton(heavyInitializer);

console.log("In this example we will be trying");
console.log("to access singleton twice before it finishes initialization.");

heavy(function(inst) {
  console.log("Access 1");
  console.log("Current property value: " + inst.property);
  console.log("Let's reassign this property");
  inst.property = "new value";
});

heavy(function(inst) {
  console.log("Access 2. You can see callbacks order is preserved.");
  console.log("Current property value: " + inst.property);
});

console.log("We made it to the end of the file. Instance is not ready yet.");

function Once() {
    return this.constructor.instance || (this.constructor.instance = this);
}

function Application(name) {
    let app = Once.call(this);

    app.name = name;

    return app;
}

如果你在上课:

class Once {
    constructor() {
        return this.constructor.instance || (this.constructor.instance = this);
    }
}

class Application extends Once {
    constructor(name) {
        super();

        this.name = name;
    }
}

测试:

console.log(new Once() === new Once());

let app1 = new Application('Foobar');
let app2 = new Application('Barfoo');

console.log(app1 === app2);
console.log(app1.name); // Barfoo

我相信这是最简单/最干净和最直观的方式，尽管它需要ECMAScript 2016 (ES7):

导出默认类Singleton { 静态实例; 构造函数(){ 如果(){ 返回实例; ｝ 这一点。State = "公爵"; 这一点。实例= this; ｝ ｝

源代码来自:adam-bien.com

如果你正在使用node.JS，那么你可以利用node.JS的缓存机制，你的单例将像这样简单:

class Singleton {
    constructor() {
        this.message = 'I am an instance';
    }
}
module.exports = new Singleton();

请注意，我们导出的不是类Singleton，而是实例Singleton()。

Node.JS将在每次需要时缓存和重用相同的对象。

更多细节请查看:Node.JS和单例模式