js单例模式

1、使用构造函数的默认属性

function A(name){
    // 如果已存在对应的实例
   if(typeof A.instance === 'object'){
       return A.instance
   }
   //否则正常创建实例
   this.name = name
    
   // 缓存
   A.instance =this
   return this
}
var a1 = new A() 
var a2= new A()
console.log(a1 === a2)//true

　2、借助闭包

var Head = (function () {
    var HeadClass = function () { }; // 声明HeadClass对象，无法在外部直接调用
    var instance; // 声明一个instance对象
    return function () {
        if (instance) { // 如果已存在 则返回instance
            return instance;
        }
        instance = new HeadClass() // 如果不存在 则new一个
        return instance;
    }
})();
var a = Head();
var b = new Head();
console.log(a===b) // true
var a = HeadClass(); // 报错,HeadClass is not defined

　3、立即执行函数

var A;
(function(name){
    var instance;
    A = function(name){
        if(instance){
            return instance
        }
         
        //赋值给私有变量
        instance = this
         
        //自身属性
        this.name = name
    }
}());
A.prototype.pro1 = "from protptype1"
 
var a1 = new A('a1') 
A.prototype.pro2 = "from protptype2"
var a2 = new A('a2')
 
console.log(a1.name)
console.log(a1.pro1)//from protptype1
console.log(a1.pro2)//from protptype2
console.log(a2.pro1)//from protptype1
console.log(a2.pro2)//from protptype2