JavaScript 实现单例模式的两种方式

export default class Model3 {
    constructor() { }
    static get instance() {
        if (!Model3._instance) {
            Object.defineProperty(Model3, "_instance", {
                value: new Model3()
            });
        }
        return Model3._instance;
    }
}

export default (function(){
    var _instance;
    return class Model1{
        static getInstance(){
            if(!_instance) _instance = new Model1();
            return _instance;
        }
    }
})();