SOURCE

//单例模式:只容许实例化一次
function single(fn){
    let instance
    let handler = {
        constructor(target, args){
            if(!instance){
                instance = Reflect.constructor(target, args)
            }
            return instance
        }
    }
    return new Proxy(fn, handler)
}
function fun (){
    console.log(111111)
}
console.log(single(fun)())