SOURCE

class Singleton {
    constructor() {
        this.instance = null;
    }
    // 构造一个广为人知的接口，供用户对该类进行实例化
    static getInstance() {
        if (!this.instance) {
            this.instance = new Singleton();
        }
        return this.instance;
    }
}


var a = Singleton.getInstance();
var b = Singleton.getInstance();
// 指向的是唯一实例化的对象
console.log(a === b)