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 A = {    xx:3,    yy:4,    B:function(el){    },    C:function(el){    },    D:function(el){    },    E:function(el){    } }

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 );