typescript 45 lines · 8 steps

A type-safe event emitter in TypeScript

How generics tie event names to their exact argument types so listeners and emits stay in sync at compile time.

Explained by highlit
Advanced generics mapped-types event-emitter type-safety observer-pattern
1type EventMap = Record<string, unknown[]>;
2 
3type Listener<Args extends unknown[]> = (...args: Args) => void;
4 
5export class TypedEmitter<Events extends EventMap> {
6 private listeners: {
7 [K in keyof Events]?: Set<Listener<Events[K]>>;
8 } = {};
9 
10 on<K extends keyof Events>(event: K, listener: Listener<Events[K]>): this {
11 (this.listeners[event] ??= new Set()).add(listener);
12 return this;
13 }
14 
15 once<K extends keyof Events>(event: K, listener: Listener<Events[K]>): this {
16 const wrapper: Listener<Events[K]> = (...args) => {
17 this.off(event, wrapper);
18 listener(...args);
19 };
20 return this.on(event, wrapper);
21 }
22 
23 off<K extends keyof Events>(event: K, listener: Listener<Events[K]>): this {
24 this.listeners[event]?.delete(listener);
25 return this;
26 }
27 
28 emit<K extends keyof Events>(event: K, ...args: Events[K]): boolean {
29 const handlers = this.listeners[event];
30 if (!handlers || handlers.size === 0) return false;
31 for (const handler of [...handlers]) {
32 handler(...args);
33 }
34 return true;
35 }
36 
37 removeAllListeners<K extends keyof Events>(event?: K): this {
38 if (event === undefined) {
39 this.listeners = {};
40 } else {
41 delete this.listeners[event];
42 }
43 return this;
44 }
45}
01 / 01
STEP 01

Walkthrough

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Three takeaways
  1. 1Mapping an event-name type to its argument tuple lets the compiler check every listener and emit for you.
  2. 2Copying the handler set before iterating avoids skipping or double-firing when handlers mutate the set mid-emit.
  3. 3A `once` listener is just a normal listener that removes itself before delegating to the real callback.

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