Update type definitions

index.d.ts

@@ -1,11 +1,45 @@
-type EventNames<T extends string | symbol | { [K in string | symbol]: any[] }> = T extends string | symbol ? T : keyof T;
-type EventArgs<T extends string | symbol | { [K in string | symbol]: any[] }, K extends EventNames<T>> = T extends string | symbol ? any[] : K extends keyof T ? T[K] : never;
+type ValidEventTypes<T = any> = string | symbol | T extends {
+  [K in keyof T]: any[] | ((...args: any[]) => void);
+}
+  ? T
+  : never;
+
+type EventNames<T extends ValidEventTypes> = T extends string | symbol
+  ? T
+  : keyof T;
+
+type Handler<
+  T extends any[] | ((...args: any[]) => R),
+  R = any
+> = T extends any[] ? (...args: T) => R : T;
+
+type EventListener<
+  T extends ValidEventTypes,
+  K extends EventNames<T>
+> = T extends string | symbol
+  ? (...args: any[]) => void
+  : K extends keyof T
+  ? Handler<T[K], void>
+  : never;
+
+type EventArgs<T extends ValidEventTypes, K extends EventNames<T>> = Parameters<
+  EventListener<T, K>
+>;
 
 /**
  * Minimal `EventEmitter` interface that is molded against the Node.js
  * `EventEmitter` interface.
  */
-declare class EventEmitter<EventTypes extends string | symbol | { [K in keyof EventTypes]: any[] } = string | symbol> {
+declare class EventEmitter<
+  EventTypes extends
+    | string
+    | symbol
+    | {}
+    | { [K in keyof EventTypes]: any[] | ((...args: any[]) => void) } =
+    | string
+    | symbol,
+  Context extends any = any
+> {
   static prefixed: string | boolean;
 
   /**
@@ -17,7 +51,9 @@ declare class EventEmitter<EventTypes extends string | symbol | { [K in keyof Ev
   /**
    * Return the listeners registered for a given event.
    */
-  listeners<T extends EventNames<EventTypes>>(event: T): Array<EventEmitter.ListenerFn<EventArgs<EventTypes, T>>>;
+  listeners<T extends EventNames<EventTypes>>(
+    event: T
+  ): Array<EventListener<EventTypes, T>>;
 
   /**
    * Return the number of listeners listening to a given event.
@@ -27,24 +63,49 @@ declare class EventEmitter<EventTypes extends string | symbol | { [K in keyof Ev
   /**
    * Calls each of the listeners registered for a given event.
    */
-  emit<T extends EventNames<EventTypes>>(event: T, ...args: EventArgs<EventTypes, T>): boolean;
+  emit<T extends EventNames<EventTypes>>(
+    event: T,
+    ...args: EventArgs<EventTypes, T>
+  ): boolean;
 
   /**
    * Add a listener for a given event.
    */
-  on<T extends EventNames<EventTypes>>(event: T, fn: EventEmitter.ListenerFn<EventArgs<EventTypes, T>>, context?: any): this;
-  addListener<T extends EventNames<EventTypes>>(event: T, fn: EventEmitter.ListenerFn<EventArgs<EventTypes, T>>, context?: any): this;
+  on<T extends EventNames<EventTypes>>(
+    event: T,
+    fn: EventListener<EventTypes, T>,
+    context?: Context
+  ): this;
+  addListener<T extends EventNames<EventTypes>>(
+    event: T,
+    fn: EventListener<EventTypes, T>,
+    context?: Context
+  ): this;
 
   /**
    * Add a one-time listener for a given event.
    */
-  once<T extends EventNames<EventTypes>>(event: T, fn: EventEmitter.ListenerFn<EventArgs<EventTypes, T>>, context?: any): this;
+  once<T extends EventNames<EventTypes>>(
+    event: T,
+    fn: EventListener<EventTypes, T>,
+    context?: Context
+  ): this;
 
   /**
    * Remove the listeners of a given event.
    */
-  removeListener<T extends EventNames<EventTypes>>(event: T, fn?: EventEmitter.ListenerFn<EventArgs<EventTypes, T>>, context?: any, once?: boolean): this;
-  off<T extends EventNames<EventTypes>>(event: T, fn?: EventEmitter.ListenerFn<EventArgs<EventTypes, T>>, context?: any, once?: boolean): this;
+  removeListener<T extends EventNames<EventTypes>>(
+    event: T,
+    fn?: EventListener<EventTypes, T>,
+    context?: Context,
+    once?: boolean
+  ): this;
+  off<T extends EventNames<EventTypes>>(
+    event: T,
+    fn?: EventListener<EventTypes, T>,
+    context?: Context,
+    once?: boolean
+  ): this;
 
   /**
    * Remove all listeners, or those of the specified event.
@@ -58,7 +119,7 @@ declare namespace EventEmitter {
   }
 
   export interface EventEmitterStatic {
-    new<EventTypes extends string | symbol | { [K in keyof EventTypes]: any[] } = string | symbol>(): EventEmitter<EventTypes>;
+    new <EventTypes extends ValidEventTypes>(): EventEmitter<EventTypes>;
   }
 
   export const EventEmitter: EventEmitterStatic;