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Javascript 中的单例模式

所谓单例模式，即是限定一个对象生成的实例数只有一个，这个唯一的实例就是单例。

举个最简单的例子：

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var user = {  
    firstName: 'John',
    lastName: 'Doe',
    sayName: function() {
        return this.firstName + ' ' + this.lastName;
    }
};

这个user就是一个单例

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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 = "Im also private";

    return {

      // Public methods and variables
      publicMethod: function () {
        console.log( "The public can see me!" );
      },

      publicProperty: "I am also public"
    };

  };

  return {

    // Get the Singleton instance if one exists
    // or create one if it doesn't
    getInstance: function () {

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

      return instance;
    }

  };

})();

// Usage:

var singleA = mySingleton.getInstance();  
var singleB = mySingleton.getInstance();  
console.log( singleA === singleB ); // true

参考：

http://robdodson.me/javascript-design-patterns-singleton/