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2026-08-31 Easy

Typed In-Memory Event Emitter with Listener Registry

You're building the event bus for a real-time dashboard application. UI components subscribe to typed events (user login, metric update, alert fired), and the compiler must guarantee that every listener receives exactly the right payload shape — no casting, no guessing.

Goals

Define AppEventMap with three precisely typed event entries including a union-typed severity field.
Create the ListenerRegistry mapped type so each event key maps to the correct array of typed listeners.
Implement on(), off(), emit(), and listenerCount() using generic constraints so the compiler enforces payload shapes at every call site.
Initialise the registry in the constructor so every event slot starts as an empty array without using `any`.

          
          challenge.ts

// Key types you must define and use:

export type AppEventMap = {
  "user:login":    { userId: string; timestamp: number };
  "metric:update": { metricId: string; value: number; unit: string };
  "alert:fired":   { alertId: string; severity: "low" | "medium" | "high"; message: string };
};

export type Listener<E extends keyof AppEventMap> = (
  payload: AppEventMap[E]
) => void;

// Helper registry — maps every event name to its listener array:
type ListenerRegistry = { [E in keyof AppEventMap]: Listener<E>[] };

export class EventEmitter {
  private registry: ListenerRegistry; // no `any` allowed!

  on<E extends keyof AppEventMap>(event: E, listener: Listener<E>): void { /* TODO */ }
  off<E extends keyof AppEventMap>(event: E, listener: Listener<E>): void { /* TODO */ }
  emit<E extends keyof AppEventMap>(event: E, payload: AppEventMap[E]): void { /* TODO */ }
  listenerCount<E extends keyof AppEventMap>(event: E): number { /* TODO */ return 0; }
}

Hints (click to reveal)

Hints

The ListenerRegistry mapped type looks like `{ [E in keyof AppEventMap]: Listener<E>[] }` — each slot's element type depends on its own key.
To seed the registry in the constructor without `any`, cast the accumulator through `as ListenerRegistry` only once after building it with `Object.fromEntries` — or simply assign each key manually.
Inside off(), use `Array.prototype.indexOf` or `.filter` to remove the exact listener reference; remember that function identity matters, not deep equality.

Useful resources

StackBlitz

CodeSandbox

Or clone locally

git clone -b challenge/2026-08-31 https://github.com/niltonheck/typedrop.git