safe-event-emitter.ts

class EventEmitter<T extends object> {
    listeners: { [K in keyof T]?: Set<(data: T[K]) => void> } = {};
    on<K extends keyof T>(name: K, cb: (data: T[K]) => void) {
        const listeners = this.listeners[name] ?? new Set();
        listeners.add(cb);
        this.listeners[name] = listeners;
        return () => listeners.delete(cb);
    }
    emit<K extends keyof T>(name: K, data: T[K]) {
        const listeners = this.listeners[name] ?? new Set();
        listeners.forEach((fn) => fn(data));
    }
}


const eventEmitter = const EventEmitter<{ data: number, error:  Error }>()
    
eventEmitter.on('data', data => ...) // data: number
eventEmitter.on('error', err => ...) // err: Error

eventEmitter.emit('data', "oops") // ❌ expected number
eventEmitter.emit('error', new Error("✅"))