Add type to retrieve valid events of an EventTarget

Search Terms

EventListener, EventMap, EventTarget

Suggestion

There should a type to retrieve valid event names and listeners for a given EventTarget. Window has the following method:

addEventListener<K extends keyof WindowEventMap>(type: K, listener: (this: Window, ev: WindowEventMap[K]) => any, options?: boolean | AddEventListenerOptions): void;

Every EventTarget should have its own corresponding EventMap (even if no special event types are available for a given target, thus, resulting in an empty object). The EventMap of a given EventTarget could be retrieved as follows:

type WindowEventMap = EventMap<Window>;
type DocumentEventMap = EventMap<Document>;
type AudioNodeEventMap = EventMap<AudioNode>; // Empty interface
// ...

Use Cases

I tried creating a function which encapsulates an event for easier lifecycle management with React Hooks:

export function managedEventListener<T extends EventTarget, K extends string>(
  target: T,
  type: K,
  callback: EventListener,
  options?: AddEventListenerOptions,
) {
  target.addEventListener(type, callback, options);
  return () => {
    target.removeEventListener(type, callback, options);
  };
}

Unfortunately, EventListener gives no proper IntelliSense and I had to use the as EventListener syntax like below, as suggested in #28357:

useEffect(
  () =>
    managedEventListener(window, 'deviceorientation', ((
      event: DeviceOrientationEvent,
    ) => {
      setOrientation(event);
    }) as EventListener),
  [],
);

My goal was to simplify the syntax to the following, with proper type inference:

useEffect(
  () =>
    managedEventListener(window, 'deviceorientation', event => {
      setOrientation(event);
    }),
  [],
);

Using conditional types, I was able to achieve the syntax above by replacing EventListener with a specialized EventListenerCallback type which extracts values from the 2 most commonly used event maps, namely WindowEventMap and DocumentEventMap:

type ExtractFrom<T, K> = K extends keyof T ? T[K] : never;
export type EventListenerCallback<T, K> = T extends Window
  ? (event: ExtractFrom<WindowEventMap, K>) => any
  : (T extends Document
      ? (event: ExtractFrom<DocumentEventMap, K>) => any
      : EventListener);

The new code for managedEventListener was born:

export function managedEventListener<T extends EventTarget, K extends string>(
  target: T,
  type: K,
  callback: EventListenerCallback<T, K>,
  options?: AddEventListenerOptions,
) {
  target.addEventListener(type, callback, options);
  return () => {
    target.removeEventListener(type, callback, options);
  };
}

Examples

The code above could be greatly simplified by introducing the aforementioned EventMap<EventTarget> type:

export function managedEventListener<T extends EventTarget, K extends keyof EventMap<T>>(
  target: T,
  type: K,
  callback: (this: T, ev: EventMap<T>[K]) => any,
  options?: AddEventListenerOptions,
) {
  target.addEventListener(type, callback, options);
  return () => {
    target.removeEventListener(type, callback, options);
  };
}

Checklist

My suggestion meets these guidelines:

• This wouldn’t be a breaking change in existing TypeScript/JavaScript code
• This wouldn’t change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn’t a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript’s Design Goals.

[jsejcksn]

Also, EventTarget should be able to accept a type argument which is an EventMap, so that classes can extend EventTargetand the corresponding listeners will recognize the appropriate types and events.

Here’s an example:

type SpellCastEvent = CustomEvent<{
  id: number;
  effect: string;
  name: string;
}> & {
  type: 'spell_cast';
};

type SpellcasterEventMap = {
  spell_cast: SpellCastEvent;
};

class Spellcaster extends EventTarget<SpellcasterEventMap> {
  constructor() {
    super();
    // ...
  }
}

function spellEventHandler (event: SpellCastEvent) {
  console.log(`The ${event.detail.name} spell was cast.`);
}

const spellcaster = new Spellcaster();

spellcaster.addEventListener('spell_cast', spellEventHandler);

Right now, this causes an error when calling spellcaster.addEventListener(), and has to be addressed with a type assertion in the handler like so:

function spellEventHandler (event: Event) {
  const ev = (event as SpellCastEvent);
  console.log(`The ${ev.detail.name} spell was cast.`);
}

You can see the errors I’m describing in #39425.

[pushkine]

The first solution coming to my mind is related to #26591

type EventTypes<El extends EventTarget> = Parameters<El["addEventListener"]>[0];

Yet there’s also the fact that string | "custom_event_type" collapses to string I can’t really think of a good solution but in the mean time here’s my usual goto

type EventMap<T extends EventTarget> = T extends MediaQueryList
	? MediaQueryListEventMap
	: T extends Document
	? DocumentEventMap
	: T extends Window
	? WindowEventMap
	: HTMLElementEventMap;
type EventTypes<T extends EventTarget> = keyof EventMap<T> & string;
type EventValue<T extends EventTarget, K extends EventTypes<T>> = Extract<EventMap<T>[K], Event>;

function listen<T extends EventTarget, K extends EventTypes<T>>(
	element: T,
	type: K,
	listener: (this: T, ev: EventValue<T, K>) => void,
	opts: AddEventListenerOptions
) {
	element.addEventListener(type, listener, opts);
	return element.removeEventListener.bind(element, type, listener, opts);
}