jsはイベント処理を実現し、購読者モードを実現する.

class event {
    construct(){
        this.event = {};
    }
    function once(eventName, cb) = {
        this.on(eventName,(...args) => {
            cb(...args);
            this.off(eventName);
        })；
    }
    function emit(eventName, ...args) {
        if(this.event.hasOwnPropety(eventName)){
            for(let cb of this.event[eventName]){
                cb(...args);
            }
        }
    }
    function on(eventName, cb) {
        if (!this.event.hasOwnPropety(eventName)){
            this.event[eventName] = [];
        }
        this.event[eventName].push(cb);
    }
    function off(eventName) {
        delete this.event[eventName];
    }
}