SOURCE

function Event(type, fn) {
    this.cache = {};
    this.add = (type, fn) => {
        if (this.cache[type]) {
            this.cache[type].push(fn)
        } else {
            this.cache[type] = [fn];
        }
    }
    this.on = (type) => {
        this.cache[type] && this.cache[type].forEach(listener => {
            listener();
        });
    }
    this.off = (type, fn) => {
        if (this.cache[type]) {
            const index = this.cache[type].findIndex(item => item === fn);
            this.cache[type].splice(index, 1)
            console.log("type move");
        }
    }

}

const event = new Event();

const clickFn = () => {
    console.log("click on");
}
event.add("click", clickFn)
event.on("click");
event.off("click", clickFn);