Can event emitters be functional?

Question

For instance, array.map accepts a callback that returns a value.

const newArray = oldArray.map(item => doSomethingTo(item));

Promises also accept callbacks that return a value.

const requestPromise = $.ajax(...).then(res => doSomethingTo(res));

But the way I see most event emitters, they seem non-functional by design.

const emitter = new EventEmitter();

// emitter.on mutates the emitter to register event
// returns nothing in most cases.

emitter.on('SOME_EVENT', event => {

  // Do something entirely irrelevant to the emitter.
  // Do something that mutates something outside the callback.
  // Returns nothing unlike array.map and promise callbacks.
  // Return value is irrelevant to the caller.

});

// Executes callbacks but return values are irrelevant.
// Returns nothing.

emitter.trigger('SOME_EVENT');

So if my understanding of functional programming is correct, event emitter callbacks aren't pure nor transparent if this is the case.

Can event emitters be written in a more functional way? Or is this a limitation of the mechanism? Are event-driven and functional incompatible? How are event emitters done in other, more functional languages?

Answer 1

In the Redux world (which took it from the functional programming world), it works like this:

function reducer(state, action) {
   // action is the "event"
   switch (action.type) {
       case 'SOME_EVENT':
           return {...state, value: 'modified'}
       default:
           return state;
   }
}

This allows your event handling to be pure. All your function does it take the current state and the event, and return a new event.