feat: 🎸 improve useObservable() type annotations

Author: streamich

src/useObservable.ts

@@ -1,6 +1,16 @@
 import { useEffect, useState } from 'react';
 
-const useObservable = <T>(observable$, initialValue?: T): T | undefined => {
+export interface Observable<T> {
+  subscribe: (
+    listener: (value: T) => void
+  ) => {
+    unsubscribe: () => void;
+  };
+}
+
+function useObservable<T>(observable$: Observable<T>): T | undefined;
+function useObservable<T>(observable$: Observable<T>, initialValue: T): T;
+function useObservable<T>(observable$: Observable<T>, initialValue?: T): T | undefined {
   const [value, update] = useState<T | undefined>(initialValue);
 
   useEffect(() => {
@@ -9,6 +19,6 @@ const useObservable = <T>(observable$, initialValue?: T): T | undefined => {
   }, [observable$]);
 
   return value;
-};
+}
 
 export default useObservable;