SOURCE

// 单例模式就是共享一块内存，new 一个实例不会再开辟新内存
class Modal {
    constructor() {
        this.dom = null;
        if (!Modal.instance) {
            this.dom = document.createElement("div");
            this.dom.innerHTML = "this is a modal!"
            this.dom.id = 'modal';
            this.dom.style.display = "none";
            this.dom.show = this.show.bind(this);
            this.dom.hide = this.hide.bind(this);
            document.body.appendChild(this.dom);
            Modal.instance = this.dom;
        }
        return Modal.instance;
    }

    show() {
        this.dom.style.display = 'block';
    }

    hide() {
        this.dom.style.display = 'none';
    }

}

document.getElementById("open").addEventListener("click", function () {
    let modal = new Modal();
    modal.show();
})
document.getElementById("close").addEventListener("click", function () {
    let modal = new Modal();
    modal.hide();
})