feat(readonly): implement readonly operator

Improved implementation when argument is already derived.

src/readonly/index.ts

@@ -4,6 +4,10 @@ export function readonly<T extends unknown>(source: Store<T>): Store<T>;
 export function readonly<T extends unknown>(source: Event<T>): Event<T>;
 
 export function readonly<T extends unknown>(source: Store<T> | Event<T>) {
+  if (!is.targetable(source)) {
+    return source;
+  }
+
   if (is.store(source)) {
     return source.map((value) => value, { skipVoid: false });
   }

src/readonly/readme.md

@@ -37,6 +37,8 @@ result = readonly(event);
 
 - `result: Store<T>|Event<T>`
 
+Note: if passed argument is already derived, then argument returns as-is.
+
 ### Example
 
 ```ts

src/readonly/readonly.test.ts

@@ -14,3 +14,19 @@ it('should convert event to readonly event', () => {
 
   expect(is.targetable(result)).toBe(false);
 });
+
+it('should return store as-is if it is already derived', () => {
+  const $store = createStore({});
+  const $mapped = $store.map((state) => state);
+  const $result = readonly($mapped);
+
+  expect($result).toBe($mapped);
+});
+
+it('should return event as-is if it is already derived', () => {
+  const event = createEvent();
+  const mapped = event.map((value) => value);
+  const result = readonly(mapped);
+
+  expect(result).toBe(mapped);
+});