我不确定我是否同意用模块模式来代替单例模式。我经常看到单例对象在完全没有必要的地方被使用和滥用，我确信模块模式填补了程序员使用单例对象的许多空白。然而，模块模式不是单例的。

模块模式:

var foo = (function () {
    "use strict";
    function aPrivateFunction() {}
    return { aPublicFunction: function () {...}, ... };
}());

在模块模式中初始化的所有内容都在声明Foo时发生。此外，模块模式可用于初始化构造函数，然后可以多次实例化构造函数。虽然模块模式是许多工作的正确工具，但它并不等同于单例模式。

单例模式:

简式

var Foo = function () {
    "use strict";
    if (Foo._instance) {
        // This allows the constructor to be called multiple times
        // and refer to the same instance. Another option is to
        // throw an error.
        return Foo._instance;
    }
    Foo._instance = this;
    // Foo initialization code
};
Foo.getInstance = function () {
    "use strict";
    return Foo._instance || new Foo();
}

长格式，使用模块模式

var Foo = (function () {
    "use strict";
    var instance; //prevent modification of "instance" variable
    function Singleton() {
        if (instance) {
            return instance;
        }
        instance = this;
        //Singleton initialization code
    }
    // Instance accessor
    Singleton.getInstance = function () {
        return instance || new Singleton();
    }
    return Singleton;
}());

在我提供的两个版本的单例模式中，构造函数本身都可以用作访问器:

var a,
    b;
a = new Foo(); // Constructor initialization happens here
b = new Foo();
console.log(a === b); //true

如果你不习惯这样使用构造函数，你可以在If (instance)语句中抛出一个错误，并坚持使用长形式:

var a,
    b;
a = Foo.getInstance(); // Constructor initialization happens here
b = Foo.getInstance();
console.log(a === b); // true

我还应该提到，单例模式很适合隐式构造函数模式:

function Foo() {
    if (Foo._instance) {
        return Foo._instance;
    }
    // If the function wasn't called as a constructor,
    // call it as a constructor and return the result
    if (!(this instanceof Foo)) {
        return new Foo();
    }
    Foo._instance = this;
}
var f = new Foo(); // Calls Foo as a constructor
-or-
var f = Foo(); // Also calls Foo as a constructor

我喜欢将单例模式和模块模式结合起来使用，并将初始化时分支与Global NS检查结合起来使用，并将它们包装在一个闭包中。

在初始化单例后环境不会改变的情况下，使用立即调用的object-literal来返回一个充满实用程序的模块，该模块将持续一段时间。

我没有传递任何依赖项，只是在它们自己的小世界中调用单例对象——唯一的目标是:为事件绑定/解绑定创建一个实用程序模块(设备方向/方向变化也可以在这种情况下工作)。

window.onload = ( function( _w ) {
    console.log.apply( console, ['it', 'is', 'on'] );
    ( {
        globalNS : function() {
            var nameSpaces = ["utils", "eventUtils"],
                nsLength = nameSpaces.length,
                possibleNS = null;

            outerLoop:
            for ( var i = 0; i < nsLength; i++ ) {
                if ( !window[nameSpaces[i]] ) {
                    window[nameSpaces[i]] = this.utils;
                    break outerLoop;
                };
            };
        },
        utils : {
            addListener : null,
            removeListener : null
        },
        listenerTypes : {
            addEvent : function( el, type, fn ) {
                el.addEventListener( type, fn, false );
            },
            removeEvent : function( el, type, fn ) {
                el.removeEventListener( type, fn, false );
            },
            attachEvent : function( el, type, fn ) {
                el.attachEvent( 'on'+type, fn );
            },
            detatchEvent : function( el, type, fn ) {
                el.detachEvent( 'on'+type, fn );
            }
        },
        buildUtils : function() {
            if ( typeof window.addEventListener === 'function' ) {
                this.utils.addListener = this.listenerTypes.addEvent;
                this.utils.removeListener = this.listenerTypes.removeEvent;
            } else {
                this.utils.attachEvent = this.listenerTypes.attachEvent;
                this.utils.removeListener = this.listenerTypes.detatchEvent;
            };
            this.globalNS();
        },
        init : function() {
            this.buildUtils();
        }
    } ).init();
} ( window ) );

我反对我的回答，看看我的另一个。

通常，模块模式(参见Christian C. Salvadó的答案)不是单例模式就足够好了。然而，单例的一个特点是它的初始化会延迟到需要对象时。模块模式缺乏这个特性。

我的命题(CoffeeScript):

window.singleton = (initializer) ->
  instance = undefined
  () ->
    return instance unless instance is undefined
    instance = initializer()

在JavaScript中编译为:

window.singleton = function(initializer) {
    var instance;
    instance = void 0;
    return function() {
        if (instance !== void 0) {
            return instance;
        }
        return instance = initializer();
    };
};

然后我可以做以下事情:

window.iAmSingleton = singleton(function() {
    /* This function should create and initialize singleton. */
    alert("creating");
    return {property1: 'value1', property2: 'value2'};
});


alert(window.iAmSingleton().property2); // "creating" will pop up; then "value2" will pop up
alert(window.iAmSingleton().property2); // "value2" will pop up but "creating" will not
window.iAmSingleton().property2 = 'new value';
alert(window.iAmSingleton().property2); // "new value" will pop up

关键是要理解闭包背后的重要性。因此，在闭包的帮助下，即使在内部函数内部的属性也将是私有的。

var Singleton = function () {
    var instance;

    function init() {

       function privateMethod() {
           console.log("private via closure");
       }

       var privateVariable = "Private Property";

       var privateRandomNumber = Math.random(); // This is also private

       return {
           getRandomNumber: function () {  // Access via getter in init call
               return privateRandomNumber;
           }
       };
    };

    return {
        getInstance: function () {

            if (!instance) {
                instance = init();
            }
            return instance;
        }
    };
};

JavaScript中的单例是使用模块模式和闭包实现的。

下面的代码是不言自明的

// Singleton example.
var singleton = (function() {
  var instance;

  function init() {
    var privateVar1 = "this is a private variable";
    var privateVar2 = "another var";

    function pubMethod() {
      // Accessing private variables from inside.
      console.log(this.privateVar1);
      console.log(this.privateVar2);
      console.log("inside of a public method");
    };
  }

  function getInstance() {
    if (!instance) {
      instance = init();
    }
    return instance;
  };

  return {
    getInstance: getInstance
  }
})();

var obj1 = singleton.getInstance();
var obj2 = singleton.getInstance();

console.log(obj1 === obj2); // Check for type and value.

这个知识是基于我正在学习Java，虽然Java和Javascript是不同的，但单例的概念和Java如何做到这一点是一样的。在我看来，JS的类样式本身是干净的，而不是var初始化。

class Singleton {
    // use hashtag which entails that the variable can only be accessed from self scope
    static #instance = null;
    static getInstance() {
        if (this.#instance === null) this.#instance = new Singleton();
        return this.#instance;
    }

    // some class property
    hello = 'world';

    // or initialize the variable in the constructor, depend on your preference
    constructor() {
        // this.hello = 'world';
    }

    /* you can also add parameters on the constructor & getInstance
     * e.g. 
     * static getInstance(param1, param2) {...new Singleton(param1, param2)}
     * constructor(param1, param2) {...}
     */

}




// this is the same code for java and normal way for singleton for class
// just use static so you can get instance


// testing the singleton
var s1,s2;
s1 = Singleton.getInstance();
s2 = Singleton.getInstance();

// you cannot access the property, immediately
if (Singleton.hello === undefined) console.log('getInstance so you can access this');

console.log(s1.hello);
// result: "world"

console.log(s2.hello);
// result: "world"


// set the value of Singleton object
s2.hello = "hi";
    console.log(s1.hello);
    // result: "hi"

    console.log(s2.hello);
    // result: "hi"
// this is just an evidence which means that they are the same even in property level

if (s1 === s2) console.log("S1 & S2 is the same object");
// result: "S1 & S2 is the same object"

// don't use something like `var s1 = new Singleton();`
// this will defeat your purpose of just (1 object), one instance of class