SOURCE

class LoginForm {
    constructor() {
        this.state = 'hide'
    }
    show() {
        if (this.state === 'show') {
            alert('已经显示')
            return
        }
        this.state = 'show'
        console.log('登录框显示成功')
    }
    hide() {
        if (this.state === 'hide') {
            alert('已经隐藏')
            return
        }
        this.state = 'hide'
        console.log('登录框隐藏成功')
    }
}
LoginForm.getInstance = (function () {  // 闭包缓存
    let instance
    return function () {
        if (!instance) {
            instance = new LoginForm()
        }
        return instance
    }
})()

let obj1 = LoginForm.getInstance()
obj1.show()

let obj2 = LoginForm.getInstance()
obj2.hide()

console.log(obj1 === obj2)