element-plus/packages/utils/types.ts

type OptionalKeys<T extends Record<string, unknown>> = {
  [K in keyof T]: T extends Record<K, T[K]>
    ? never
    : K
}[keyof T]

type RequiredKeys<T extends Record<string, unknown>> = Exclude<keyof T, OptionalKeys<T>>

type MonoArgEmitter<T, Keys extends keyof T> = <K extends Keys>(evt: K, arg?: T[K]) => void

type BiArgEmitter<T, Keys extends keyof T> = <K extends Keys>(evt: K, arg: T[K]) => void

export type EventEmitter<T extends Record<string, unknown>> =
  MonoArgEmitter<T, OptionalKeys<T>> & BiArgEmitter<T, RequiredKeys<T>>

export type AnyFunction<T> = (...args: any[]) => T