A simple naive TS event emitter, useful for making nice typed events without remembering how to do it every time

EventEmitter.ts

type EventMap = Record<string, any>;
type EventKey<T extends EventMap> = string & keyof T;
type Callback<T> = (params: T) => void;

export class EventEmitter<T extends EventMap> {
  private listeners: {
    [K in keyof EventMap]?: Array<(p: EventMap[K]) => void>;
  } = {};

  on<K extends EventKey<T>>(key: K, fn: Callback<T[K]>) {
    this.listeners[key] = (this.listeners[key] || []).concat(fn);
  }

  off<K extends EventKey<T>>(key: K, fn: Callback<T[K]>) {
    this.listeners[key] = (this.listeners[key] || []).filter((f) => f !== fn);
  }

  emit<K extends EventKey<T>>(key: K, data: T[K]) {
    (this.listeners[key] || []).forEach(function (fn: Callback<T[K]>) {
      fn(data);
    });
  }
}