const Emit = () => {
    let listeners = {}

    const on = (eventName, listenerCb) => {
        if(listeners.hasOwnProperty(eventName)) {
            console.error('Already exist')
            return
        }

        if(typeof eventName !== 'string') {
            console.error('Event name must be a string!')
            return
        }

        if(typeof listenerCb !== 'function') {
            console.error('Callback must be a function!')
            return
        }

        listeners[eventName] = listenerCb
    }

    const off = (eventName) => {
        if(listeners.hasOwnProperty(eventName))
            delete listeners[eventName]
    }

    const emit = (eventName) => {
        if(!listeners.hasOwnProperty(eventName)) {
            console.log(`${eventName} is not registered!`)
            return
        }

        listeners[eventName]()
    }

    const getListener = (eventName) => {
        if(listeners.hasOwnProperty(eventName)) {
            return listeners[eventName]
        } else { return false }
    }

    return {
        on, off, emit, getListener
    }

}






const myEmitter = Emit()

myEmitter.on('click', () => { console.log('======== im fired ========') })
myEmitter.on('scroll', () => { console.log('scrolled') })

let myInterval = setInterval(() => {
    if(myEmitter.getListener('scroll')) {
        myEmitter.emit('scroll')
    } else {
        console.log('scroll STOP')
        clearInterval(myInterval)
    }
}, 200)

setTimeout(() => {
    myEmitter.emit('click')
    myEmitter.off('click')
}, 2000)

setTimeout(() => {
    myEmitter.emit('click')
    myEmitter.off('scroll')
}, 4000)