Extending Event Emitter with Flow Types

This repo serves as an answer to a stackoverflow question on how to extend EventEmitter and still let Flow know what kind of arguments were passed to the event emitter.

Original question:

As an example, let's say I have a class that only emits three possible events – 'pending' or 'success' or 'failure'. Additionally, the type of the argument received in the eventHandler depends on which event was emitted –

if 'pending', the eventHandler receives no argument

if 'success', the eventHandler receives a number

if 'failure', the eventHandler receives an Error

Here is how I tried to model that:

// @flow

import EventEmitter from 'events'

type CustomEventObj = {| pending: void, success: number, error: Error |}

declare class MyEventEmitter extends EventEmitter { on<K: $Keys>( eventName: K, eventHandler: ( e: $ElementType<CustomEventObj, K>, ...args: Array ) => void ): this }

However, this results in an error like so:

Error ┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈┈ test.js:12:3

Cannot extend EventEmitter [1] with MyEventEmitter because an indexer property is missing in CustomEventObj [2] in the first argument of property on.
[1]  3│ import EventEmitter from 'events'
      :
     8│   error: Error
     9│ |}
    10│
    11│ declare class MyEventEmitter extends EventEmitter {
[2] 12│   on<K: $Keys<CustomEventObj>>(
    13│     eventName: K,
    14│     eventHandler: (
    15│       e: $ElementType<CustomEventObj, K>,
    16│       ...args: Array<any>
    17│     ) => void
    18│   ): this
    19│ }
    20│
I don't want to have an indexer property on CustomEventObj because wouldn't that kill the point of only having 3 possible events?

Any help would be appreciated.

jameskraus/extending-event-emitter-with-flow

Extending Event Emitter with Flow Types