function Person(){ 
  this.createTime=new Date(); 
} 

var instance=new Person(); 
function getInstance(){ 
  return instance; 
} 

var instance 
function getInstance(){ 
  if(!instance){ 
    instance=new Person(); 
  } 
  return instance; 
} 

var getInstance(){ 
var instance; 
return function(){ 
    if(!instance){ 
      instance=new Person(); 
    } 
    return instance; 
  } 
}(); 

function Person(){ 
  //如果已经缓存了实例，则直接返回缓存的实例 
  if(typeof Person.instance==='object'){ 
    return Person.instance; 
  } 
  this.createTime=new Date(); 
  //缓存实例 
  Person.instance=this; 
  return this; 
} 

function Person(){ 
  //缓存实例 
  var instance=this; 
  this.createTime=new Date(); 
  //重写构造函数 
  Person=function(){ 
    return instance; 
  } 
} 

//向原型添加属性 
Person.prototype.prop1=true; 
var p1=new Person(); 
//在创建初始化对象后，再次向该原型添加属性 
Person.prototype.prop2=true; 
var p2=new Person(); 

//开始测试 
console.log(p1.prop1);//结果为true 
console.log(p2.prop1);//结果为true 

console.log(p1.prop2);//结果为undefined 
console.log(p2.prop2);//结果为undefined 

console.log(p1.constructor===Person);//结果为false 
console.log(p2.constructor===Person);//结果为false 

function Person(){ 
  //缓存实例 
  var instance=this; 
  //重写构造函数 
  Person=function(){ 
    return instance; 
  } 
  //保留原型属性 
  Person.prototype=this; 
  //实例 
  instance=new Person(); 
  //重置构造函数引用 
  instance.constructor=Person; 

  //其他初始化 
  instance.createTime=new Date(); 
 
  return instance; 
} 

/* Singleton with Private Members, step 3. */

MyNamespace.Singleton = (function() {
 // Private members.
 var privateAttribute1 = false;
 var privateAttribute2 = [1, 2, 3];

 function privateMethod1() {
  ...
 }
 function privateMethod2(args) {
  ...
 }

 return { // Public members.
  publicAttribute1: true,
  publicAttribute2: 10,

  publicMethod1: function() {
   ...
  },
  publicMethod2: function(args) {
   ...
  }
 };
})();

/* General skeleton for a lazy loading singleton, step 1. */

MyNamespace.Singleton = (function() {

 function constructor() { // All of the normal singleton code goes here.
  // Private members.
  var privateAttribute1 = false;
  var privateAttribute2 = [1, 2, 3];

  function privateMethod1() {
   ...
  }
  function privateMethod2(args) {
   ...
  }

  return { // Public members.
   publicAttribute1: true,
   publicAttribute2: 10,

   publicMethod1: function() {
    ...
   },
   publicMethod2: function(args) {
    ...
   }
  }
 }

})();

/* General skeleton for a lazy loading singleton, step 2. */

MyNamespace.Singleton = (function() {

 function constructor() { // All of the normal singleton code goes here.
  ...
 }

 return {
  getInstance: function() {
   // Control code goes here.
  }
 }
})();

/* General skeleton for a lazy loading singleton, step 3. */

MyNamespace.Singleton = (function() {

 var uniqueInstance; // Private attribute that holds the single instance.

 function constructor() { // All of the normal singleton code goes here.
  ...
 }

 return {
  getInstance: function() {
   if(!uniqueInstance) { // Instantiate only if the instance doesn't exist.
    uniqueInstance = constructor();
   }
   return uniqueInstance;
  }
 }
})();

/* SimpleXhrFactory singleton, step 1. */

var SimpleXhrFactory = (function() {

 // The three branches.
 var standard = {
  createXhrObject: function() {
   return new XMLHttpRequest();
  }
 };
 var activeXNew = {
  createXhrObject: function() {
   return new ActiveXObject('Msxml2.XMLHTTP');
  }
 };
 var activeXOld = {
  createXhrObject: function() {
   return new ActiveXObject('Microsoft.XMLHTTP');
  }
 };

})();

/* SimpleXhrFactory singleton, step 2. */

var SimpleXhrFactory = (function() {

 // The three branches.
 var standard = {
  createXhrObject: function() {
   return new XMLHttpRequest();
  }
 };
 var activeXNew = {
  createXhrObject: function() {
   return new ActiveXObject('Msxml2.XMLHTTP');
  }
 };
 var activeXOld = {
  createXhrObject: function() {
   return new ActiveXObject('Microsoft.XMLHTTP');
  }
 };

 // To assign the branch, try each method; return whatever doesn't fail.
 var testObject;
 try {
  testObject = standard.createXhrObject();
  return standard; // Return this if no error was thrown.
 }
 catch(e) {
  try {
   testObject = activeXNew.createXhrObject();
   return activeXNew; // Return this if no error was thrown.
  }
  catch(e) {
   try {
    testObject = activeXOld.createXhrObject();
    return activeXOld; // Return this if no error was thrown.
   }
   catch(e) {
    throw new Error('No XHR object found in this environment.');
   }
  }
 }

})();