js实现单例

单例无外乎每次调用返回的都是同一个对象，可以利用闭包实现

　　　　　　　function Singleton(){
                this.data = "singleton";
            }
            Singleton.getInstance = (function(){
                var instance;
                return function(){
                    instance = instance ? instance:new Singleton;
                    return instance;
                }
            })();
            
            var sa = Singleton.getInstance();
            var sb = Singleton.getInstance();
            console.log(sa === sb);
            console.log(sa.data);

输出结果

这种方式比较简单，Singleton类暴露全局，直接使用Singleton.getInstance()即可。

还有种情况，假设不想暴露Singleton，可以

　　　　　　　var SingletonA = (function(){
                var instance = null;
                function Singleton(){
                    this.init();
                }
                Singleton.prototype.init = function(){
                    this.data = "SingletonA";
                }
                return function(){
                    if(instance == null){
                        instance = new Singleton();
                    }
                    return instance;
                };
            })();
            var b = SingletonA();
            var c = SingletonA();
            console.log(b === c);
            console.log(b.data);

输出

还有一种

　　　　　　　var SingletonF = function(){
                if(SingletonF.instance){
                    console.log("不能new");
                }
                this.name = "gggg";
            };
            SingletonF.getInstance = function(name){
                if(SingletonF.instance == null){
                    SingletonF.instance = new SingletonF();
                }
                return SingletonF.instance;
            }
            var s1 = SingletonF.getInstance("ccc");
            var s2 = SingletonF.getInstance("def");
            
            console.log(s1 === s2);
            console.log(s1.name);

写法基本上大同小异，只要保证instance唯一就行了