Declaring events in a TypeScript class which extends EventEmitter

Question

I have a class extends EventEmitter that can emit event hello. How can I declare the on method with specific event name and listener signature?

class MyClass extends events.EventEmitter {

  emitHello(name: string): void {
    this.emit('hello', name);
  }

  // compile error on below line
  on(event: 'hello', listener: (name: string) => void): this;
}

Answer 1

to extend @SergeyK's answer, with this you can get type-checking and completion on both emit and on functions without repeating event types.

1. Define event listener signatures for each event type:

interface MyClassEvents {
  'add': (el: string, wasNew: boolean) => void;
  'delete': (changedCount: number) => void;
}

2. Declare interface which constructs types for MyClass, based on EventListeners (MyClassEvents) function signature:

declare interface MyClass {
  on<U extends keyof MyClassEvents>(
    event: U, listener: MyClassEvents[U]
  ): this;

  emit<U extends keyof MyClassEvents>(
    event: U, ...args: Parameters<MyClassEvents[U]>
  ): boolean;
}

3. Simply define you class extending EventEmitter:

class MyClass extends EventEmitter {
  constructor() {
    super();
  }
}

---

Now you will get type checking for on and emit functions:

Unfortunately you will get completion and type-checking only on those two functions (unless you define more functions inside MyClass interface).

To get more generic solution, you can use this package. note: it adds no runtime overhead.

import { TypedEmitter } from 'tiny-typed-emitter';

interface MyClassEvents {
  'add': (el: string, wasNew: boolean) => void;
  'delete': (changedCount: number) => void;
}

class MyClass extends TypedEmitter<MyClassEvents> {
  constructor() {
    super();
  }
}

Answer 2

Here's what I was able to figure out. Overriding the default function with a generic!

interface IEmissions {
  connect: () => void
  test: (property: string) => void
}

class MyClass extends events.EventEmitter {
  private _untypedOn = this.on
  private _untypedEmit = this.emit
  public on = <K extends keyof IEmissions>(event: K, listener: IEmissions[K]): this => this._untypedOn(event, listener)
  public emit = <K extends keyof IEmissions>(event: K, ...args: Parameters<IEmissions[K]>): boolean => this._untypedEmit(event, ...args)

  this.emit('test', 'Testing') // This will be typed for you!
}

// Example:
const inst = new MyClass()
inst.on('test', info => console.log(info)) // This will be typed!

Answer 3

You can use typed event emitter package for this.

eg:

import { EventEmitter } from 'tsee';

const events = new EventEmitter<{
    foo: (a: number, b: string) => void,
}>();

// foo's arguments is fully type checked
events.emit('foo', 123, 'hello world');

This package also provide interfaces & some utils.

Answer 4

I really liked @Binier's answer and especially the generic solution offered by tiny-typed-emitter. As an alternative, I wrote up this pure-typescript version:

type EmittedEvents = Record<string | symbol, (...args: any) => any>;

export declare interface TypedEventEmitter<Events extends EmittedEvents> {
  on<E extends keyof Events>(
    event: E, listener: Events[E]
  ): this;

  emit<E extends keyof Events>(
    event: E, ...args: Parameters<Events[E]>
  ): boolean;
}

export class TypedEventEmitter<Events extends EmittedEvents> extends EventEmitter {}

It's used similarly:

type MessageSocketEvents = {
  'message': (json: object) => void;
  'close': () => void;
};

export class MessageSocket extends TypedEventEmitter<MessageSocketEvents> {
  ...
}

Answer 5

Use the official typing package for the events library:

npm install events @types/events