Typesafe Event Emitter

Conventionally in Node.js and traditional JavaScript you have a single event emitter. This event emitter internally tracks listener for different event types e.g.

const emitter = new EventEmitter();
// Emit:
emitter.emit('foo', foo);
emitter.emit('bar', bar);
// Listen:
emitter.on('foo', (foo)=>console.log(foo));
emitter.on('bar', (bar)=>console.log(bar));

Essentially EventEmitter internally stores data in the form of mapped arrays:

{foo: [fooListeners], bar: [barListeners]}

Instead, for the sake of event type safety, you can create an emitter per event type:

const onFoo = new TypedEvent<Foo>();
const onBar = new TypedEvent<Bar>();
// Emit:
onFoo.emit(foo);
onBar.emit(bar);
// Listen:
onFoo.on((foo)=>console.log(foo));
onBar.on((bar)=>console.log(bar));

This has the following advantages:

  • The types of events are easily discoverable as variables.

  • The event emitter variables are easily refactored independently.

  • Type safety for event data structures.

Reference TypedEvent

export interface Listener<T> {
(event: T): any;
}
export interface Disposable {
dispose();
}
/** passes through events as they happen. You will not get events from before you start listening */
export class TypedEvent<T> {
private listeners: Listener<T>[] = [];
private listenersOncer: Listener<T>[] = [];
on = (listener: Listener<T>): Disposable => {
this.listeners.push(listener);
return {
dispose: () => this.off(listener)
};
}
once = (listener: Listener<T>): void => {
this.listenersOncer.push(listener);
}
off = (listener: Listener<T>) => {
var callbackIndex = this.listeners.indexOf(listener);
if (callbackIndex > -1) this.listeners.splice(callbackIndex, 1);
}
emit = (event: T) => {
/** Update any general listeners */
this.listeners.forEach((listener) => listener(event));
/** Clear the `once` queue */
if (this.listenersOncer.length > 0) {
const toCall = this.listenersOncer;
this.listenersOncer = [];
toCall.forEach((listener) => listener(event));
}
}
pipe = (te: TypedEvent<T>): Disposable => {
return this.on((e) => te.emit(e));
}
}