什么是单例模式

  单例模式 （Singleton Pattern）又称为单体模式，保证一个类只有一个实例，并提供一个访问它的全局访问点。也就是说，第二次使用同一个类创建新对象的时候，应该得到与第一次创建的对象完全相同的对象。

 简单的说就是保证一个类仅有一个实例，并提供一个访问它的全局访问点，这样的模式就叫做单例模式。

class Single {
    show() {
        console.log('我是一个单例对象')
    }
}

const s1 = new Single()
const s2 = new Single()

// false
s1 === s2

js实现

// es6
class SingletonApple {
  static instance = null;
  constructor(name, creator, products) {
    this.name = name;
    this.creator = creator;
    this.products = products;
    if (!SingletonApple.instance) {
      SingletonApple.instance = this;
    }
    return SingletonApple.instance;
  }
  //静态方法
  static getInstance(name, creator, products) {
    if (!this.instance) {
      this.instance = new SingletonApple(name, creator, products);
    }
    return this.instance;
  }
}

  console.log(SingletonApple.instance);
  let ss = new SingletonApple("苹果公司", "阿辉", 
                              ["iPhone", "iMac", "iPad", "iPod"]);
  console.log(ss);
  console.log(ss === SingletonApple.instance);
  let appleCompany = SingletonApple.getInstance("苹果公司", "乔布斯", [
    "iPhone",
    "iMac",
    "iPad",
    "iPod"
  ]);
  let copyApple = SingletonApple.getInstance("苹果公司", "阿辉", [
    "iPhone",
    "iMac",
    "iPad",
    "iPod"
  ]);
  console.log(appleCompany === copyApple); //true


// 闭包-通过自执行函数创建闭包变量
const Singleton = (function () {
    const SingleClass = function (name, creator, products) {
      this.name = name;
      this.creator = creator;
      this.products = products;
    };
    let instance = null;
    return function (name, creator, products) {
      if (!instance) {
       	 // 如果不存在 则new一个
          instance = new SingleClass(name, creator, products) 
      }
      return instance;
    }
 })()

let s1 = new Singleton("单例1", "阿辉1", [
  "iPhone",
  "iMac",
  "iPad",
  "iPod"])

let s2 = new Singleton("单例2", "阿辉2", [
  "iPhone",
  "iMac",
  "iPad",
  "iPod"])

console.log(s1 === s2)
console.log(s1)