emitter.ts

// implement class that will fallow interface and expected behaviour below 

interface EventEmitter {
    on(eventName: string, cb: VoidFunction): void
    one(eventName: string, cb: VoidFunction): void
    off(eventName: string, cb: VoidFunction): void
    emit(eventName: string): void
}

class EventEmitter implements EventEmitter {
    //
}

console.clear();

const emitter = new EventEmitter()

const cb = () => {
    console.log('foo1')
}

emitter.on('foo', cb)

emitter.one('foo', () => {
    console.log('foo2')
})

emitter.emit('foo')
// Output:
// "foo1"
// "foo2"

emitter.emit('foo')
// Output:
// "foo1"

emitter.off('foo', cb)

emitter.emit('foo')
// No output