SOURCE

class Event {
    constructor() {
        this.list = {}
    }
    on(type, fn) {
        let fns = this.list[type]
        fns = fns || []
        if(fns.indexOf(fn) === -1){
            fns.push(fn)
        }
        this.list[type] = fns
    }
    emit(type) {
        this.list[type] && this.list[type].forEach(fn => {
            fn && fn()
        })
    }
    off(type, fn) {
        let fns = this.list[type] || []
        if(fn){
            let index = fns.indexOf(fn)
            fns.splice(index, 1)
        } else {
            fns = []
        }
        this.list[type] = fns
    }
}

const event = new Event()
const fn1 = () => { console.log('11111') }
const fn2 = () => { console.log('222222') }
event.on('test', fn1)
event.on('test', fn2)
event.emit('test')
event.off('test', fn1)
console.log(event.list)