单例模式

确保一个类只有一个实例存在。并提供一个全局的访问点,而且只能通过静态方法来获取

方式一：
class Single {constructor() {if(Single._instace) throw new Error('已经存在单例')Single._instance = this}static getInstance () {if(!Single._instance) Single._instance = new Single()return Single._instance}
}

方式二：
const Single = function() {this.instance = null
}
Single.getInstance = function() {if (this.instance) {return this.instance;}return this.instance = new Single();
}

测试
const instance1 = Single.getInstance()
const instance2 = Single.getInstance()
console.log(instance1 === instance2 )

方式三： --闭包实现
class object2 {}
let Single = () => {let instance = nullconst getInstance = () => {if(!instance) instance = new object2()return instance}return getInstance
}
let singleInstance = Single()
let instance1 =singleInstance()
let instance2 = singleInstance()
console.log(instance1 === instance2)

“透明”单例模式

相比单例模式，它只能通过new方法获取

const Single = (function(){let instance;return function() {if (instance) {return instance;}return instance = this;}
})();const a = new Single();
const b = new Single();console.log(a === b);