In JavaScript, Singletons serve as a shared resource namespace which isolate implementation code from the global namespace so as to provide a single point of access for functions.

In practice, the Singleton pattern is useful when exactly one object is needed to coordinate others across a system. 

var mySingleton = (function () {
 

// Instance 存储一个单例实例的引用
var instance;

function init() {

// Singleton

// 私有的方法和变量
function privateMethod(){
    console.log( "I am private" );
}

var privateVariable = "Im also private";

return {

  // 共有的方法和变量
  publicMethod: function () {
    console.log( "The public can see me!" );
  },

  publicProperty: "I am also public"
};

};

return {

// 如果实例不存在，那么创建一个
getInstance: function () {

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

  return instance;
}

};

})();

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

var SingletonTester = (function () {
  // options: an object containing configuration options for the singleton
  // e.g var options = { name: "test", pointX: 5}; 
  function Singleton( options )  {
    // set options to the options supplied
    // or an empty object if none are provided
    options = options || {};
    // set some properties for our singleton
    this.name = "SingletonTester";
    this.pointX = options.pointX || 6;
    this.pointY = options.pointY || ; 
  }
  // our instance holder 
  var instance;
  // an emulation of static variables and methods
  var _static  = {  
    name:  "SingletonTester",
    // Method for getting an instance. It returns
    // a singleton instance of a singleton object
    getInstance:  function( options ) {   
      if( instance  ===  undefined )  {    
        instance = new Singleton( options );   
      }   
        return  instance;      
    } 
  }; 
  return  _static;
})();
var singletonTest  =  SingletonTester.getInstance({
  pointX:  5
});
// Log the output of pointX just to verify it is correct
// Outputs: 5
console.log( singletonTest.pointX );