SOURCE

class EventBus {
    constructor(){
        this.events = {}
    }

    on(key, fn, isOnce = false) {
        if (events[key] == null) {
            events[key] = []
        }

        events[key].push({ key, fn, isOnce })
    }

    once(key, fn) {
        this.on(key, fn, true)
    }

    off(key, fn) {
        const es = this.events[key]
        if (!fn) {
            this.events[key] = []
        } else {
            this.events[key] = this.events[key].filter(e => e !== fn)
        }

        console.log('off -- ', this.events[key])
    }
}

function fn1(){console.log('fn1')}
function fn2(){console.log('fn2')}
function fn3(){console.log('fn3')}

var bus = new EventBus()
bus.on('key1', fn1)