Remove type assertion:

source/event-emitter.ts

@@ -2,13 +2,21 @@ export type EventMap = Record<string, unknown>;
 
 type Listener<T> = (detail: T) => void;
 
+/**
+ * Internally permits listeners for individual event payloads to be stored
+ * in a collection typed with the union of all event payloads.
+ */
+type StoredListener<T> = {
+    bivarianceHack(detail: Readonly<T>): void;
+}['bivarianceHack'];
+
 /**
  * A listener entry.
- * @template T - The event type.
+ * @template T - The event payload type.
  */
 interface ListenerEntry<T> {
-    listener: Listener<T>;
+    listener: StoredListener<T>;
-    wrappedListener: Listener<T>;
+    wrappedListener: StoredListener<T>;
     debounceTime?: number;
     once?: boolean;
 }
@@ -50,7 +58,7 @@ export class EventEmitter<T extends EventMap> {
         };
 
         // Add the listener entry to the listeners map.
-        this.listeners.get(type)?.add(listenerEntry as ListenerEntry<T[keyof T]>);
+        this.#listeners.get(type)?.add(listenerEntry);
 
         // Return an "off" callback that can be called to stop listening for events.
         return () => this.off(type, listener);
@@ -87,7 +95,7 @@ export class EventEmitter<T extends EventMap> {
         };
 
         // Add the listener entry to the listeners map.
-        this.listeners.get(type)?.add(listenerEntry as ListenerEntry<T[keyof T]>);
+        this.#listeners.get(type)?.add(listenerEntry);
 
         // Return an "off" callback that can be called to stop listening for events.
         return () => this.off(type, listener);