More strictly typed events

[ReneWerner87]

hi and thx for this @MaximeKjaer https://github.com/socketio/socket.io/pull/3822

Observation

I wanted to use socketio together with rxjs and found out that the typescript compiler can no longer follow through the type EventEmitter https://github.com/socketio/socket.io/blame/fd9dd74eeed3fa6a15c63240df30cc3b7357102f/lib/typed-events.ts#L93 https://github.com/ReactiveX/rxjs/blob/master/src/internal/observable/fromEvent.ts#L62-L110

example code which i have reduced to the essential(click me)

import { fromEvent } from 'rxjs';
import { EventEmitter } from 'events';

interface EventsMap {
    [event: string]: any;
}
declare type EventNames<Map extends EventsMap> = keyof Map & (string | symbol);

// here the event emitter causes problems
class MyTarget<UserEvents extends EventsMap> extends EventEmitter {
// since the eventemitter adds the general event listening ("addListener", "removeListener") methods without strict types
    on<Ev extends EventNames<UserEvents>>(ev: Ev, listener: UserEvents[Ev]) {
        return this;
    }
    off<Ev extends EventNames<UserEvents>>(ev: Ev, listener: UserEvents[Ev]) {
        return this;
    }
}

// socket io event declaration approach - @see https://socket.io/docs/v4/typescript/
interface MyEvents {
    foo: (firstParam: boolean) => void;
}

const target = new MyTarget<MyEvents>();

// expect boolean type for the argument
fromEvent(target, 'foo').subscribe((firstParam) => {
    console.log('firstParam', firstParam);
});

Problem

Since the eventemitter adds the general event listening ("addListener", "removeListener") methods without strict types, the typescript compiler cannot follow the listener argument types

with EventEmitter

without EventEmitter

same also happens with the direct usage of addListener and removeListener in example https://socket.io/docs/v4/typescript/#types-for-the-server

socket.addListener('basicEmit', (a, b, c) => {})

Idea

The request would be to get rid of the native EventEmitter and replace it with the methods with a stricter one so that also the typescript of other libraries can follow and more type support for the other native methods exists

[ReneWerner87]

https://github.com/bterlson/strict-event-emitter-types/tree/master something like this for the native event emitter for the internal methods addListener and removeListener the types are already present in the interface as generic and would only have to be used for the method declarations

[ReneWerner87]

@darrachequesne @MaximeKjaer any opinions on this

[ReneWerner87]

@phiresky do you have an opinion on this? what would you do in this case?

[darrachequesne]

I think overriding the type for the addListener() and removeListener() methods in the StrictEventEmitter class here should be sufficient. @ReneWerner87 what do you think?

Note: the approach used here is even more powerful, as there is no intermediary class at runtime. Not sure if we could merge/use it.

[ReneWerner87]

Yes overwrite should be enough for now, try to create a pull-request later (just have some tasks for my project)

If the author of the other type script lib would have time and desire, he could add or merge his types

[ReneWerner87]

@darrachequesne i tried something like this https://github.com/socketio/socket.io/compare/main...ReneWerner87:socket.io:main

unfortunately do not know how I test the whole project together with my changes think i need support for the change

just found out that the method addListener is also missing in the emitter interface https://github.com/socketio/emitter/blob/4810bd7592519c522a11da3852d82e6c15eb074d/index.d.ts#L75-L179

but its there in the js code https://github.com/socketio/emitter/blob/4810bd7592519c522a11da3852d82e6c15eb074d/index.js#L42-L43

think using this other lib and combining it with yours would improve the types quite a bit

i also saw https://github.com/socketio/socket.io/pull/4817 , really interesting