Reverse inference event types from $emit

[johnsoncodehk]

I spent a lot of time on this issue, and current TS version seems can't solve it.

Problem

Given a $emit type with unknow overloads length:

type Emit = {
  (event: 'event-1', str: string): void;
  (event: 'event-2', num: number): void;
  (event: 'event-3', bool: boolean): void;
  // more events...
  (event: 'event-n', str: string, num: number, bool: boolean): void;
}

How to get the event function type with specify event name?

type EmitEvent<F, E> = ...;

type Event_1 = EmitEvent<Emit, 'event-1'>; // (str: string) => void
type Event_2 = EmitEvent<Emit, 'event-2'>; // (num: number) => void
type Event_3 = EmitEvent<Emit, 'event-3'>; // (bool: boolean) => void
// more events...
type Event_n = EmitEvent<Emit, 'event-n'>; // (str: string, num: number, bool: boolean) => void

What Now

For limited overloads length (for example 1 ~ 4), it can resolve with:

type EmitEvent<F, E> =
   F extends {
      (event: E, ...payload: infer P): infer R // match 'event n - 3'
      (...args: any): any
      (...args: any): any
      (...args: any): any
   } ? (...payload: P) => R
   : F extends {
      (event: E, ...payload: infer P): infer R // match 'event n - 2'
      (...args: any): any
      (...args: any): any
   } ? (...payload: P) => R
   : F extends {
      (event: E, ...payload: infer P): infer R // match 'event n - 1'
      (...args: any): any
   } ? (...payload: P) => R
   : F extends {
      (event: E, ...payload: infer P): infer R // match 'event n'
   } ? (...payload: P) => R
   : unknow /* or `(...payload: any[]) => any` whatever */; // match 'event n < n - 3'

(Note: Increasing the number of overloads in this way inferred time will increase exponentially, so it is not feasible to detect the number of overloads dynamically generating EmitEvent<F, E>.)

Some libraries have more than 10 events overloads, so this is still a problem to be solved.

References:

• Function argument inference only handles one overload microsoft/TypeScript#26591
• ConstructorParameters<T> cannot get correct types of overloaded constructor microsoft/TypeScript#37079 (comment)

[johnsoncodehk]

resolve with DefineComponent, thanks to @pikax !

type ExtractEmits<T> = T extends DefineComponent<
	any,
	any,
	any,
	any,
	any,
	any,
	any,
	infer E
>
	? E
	: never;
type ReturnVoid<T> = T extends (...payload: infer P) => any ? (...payload: P) => void : T;
type EmitEvent<T, E extends keyof ExtractEmits<T>> = ReturnVoid<ExtractEmits<T>[E]>;

const dd = defineComponent({
  emits: {
    "event-1"(str: string) {},
    "event-2"(num: number) {},
    "event-3"(bool: boolean) {},
    "event-n"(str: string, num: number, bool: boolean) {},
  },
});
type Event_1 = EmitEvent<typeof dd, 'event-1'>; // (str: string) => void
type Event_2 = EmitEvent<typeof dd, 'event-2'>; // (num: number) => void
type Event_3 = EmitEvent<typeof dd, 'event-3'>; // (bool: boolean) => void
type Event_n = EmitEvent<typeof dd, "event-n">; // (str: string, num: number, bool: boolean) => void