单例模式:

确保一个类只有一个实例，并提供对它的全局访问点。

单例模式将特定对象的实例数量限制为一个。这个单一实例称为单例。

定义返回唯一实例的getInstance()。 负责创建和管理实例对象。

单例对象被实现为一个即时匿名函数。函数立即执行，将它括在括号中，然后再加上两个括号。它被称为匿名，因为它没有名字。

示例程序

var Singleton = (function () { var instance; function createInstance() { var object = new Object("I am the instance"); return object; } return { getInstance: function () { if (!instance) { instance = createInstance(); } return instance; } }; })(); function run() { var instance1 = Singleton.getInstance(); var instance2 = Singleton.getInstance(); alert("Same instance? " + (instance1 === instance2)); } run()

var singleton = (function () {

    var singleton = function(){
        // Do stuff
    }
    var instance = new singleton();
    return function(){
        return instance;
    }
})();

一个没有getInstance方法的解决方案。

这也是一个单例:

function Singleton() {
    var i = 0;
    var self = this;

    this.doStuff = function () {
        i = i + 1;
        console.log('do stuff', i);
    };

    Singleton = function () { return self };
    return this;
}

s = Singleton();
s.doStuff();

下面是我实现单例模式的演练片段。这是我在面试过程中想到的，我觉得我应该在某个地方捕捉到这一点。

/*************************************************
 *     SINGLETON PATTERN IMPLEMENTATION          *
 *************************************************/

// Since there aren't any classes in JavaScript, every object
// is technically a singleton if you don't inherit from it
// or copy from it.
var single = {};


// Singleton Implementations
//
// Declaring as a global object...you are being judged!

var Logger = function() {
  // global_log is/will be defined in the GLOBAL scope here
  if(typeof global_log === 'undefined'){
    global_log = this;
  }
  return global_log;
};


// The below 'fix' solves the GLOABL variable problem, but
// the log_instance is publicly available and thus can be
// tampered with.
function Logger() {
  if(typeof Logger.log_instance === 'undefined') {
    Logger.log_instance = this;
  }

  return Logger.log_instance;
};


// The correct way to do it to give it a closure!

function logFactory() {
  var log_instance; // Private instance
  var _initLog = function() { // Private init method
    log_instance = 'initialized';
    console.log("logger initialized!")
  }
  return {
    getLog : function(){ // The 'privileged' method
      if(typeof log_instance === 'undefined') {
        _initLog();
      }
      return log_instance;
    }
  };
}


/***** TEST CODE ************************************************

// Using the Logger singleton
var logger = logFactory(); // Did I just give LogFactory a closure?

// Create an instance of the logger
var a = logger.getLog();

// Do some work
// Get another instance of the logger
var b = logger.getLog();

// Check if the two logger instances are same
console.log(a === b); // true
*******************************************************************/

同样可以在我的主旨页面上找到。

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.");

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