js设计模式： 单例模式

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 = null
    const 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);