Arg is optional

beta/src/content/apis/react/useReducer.md

@@ -7,7 +7,7 @@ title: useReducer
 `useReducer` is a React Hook that lets you add a [reducer](/learn/extracting-state-logic-into-a-reducer) to your component.
 
 ```js
-const [state, dispatch] = useReducer(reducer, initialArg, init)
+const [state, dispatch] = useReducer(reducer, initialArg, init?)
 ```
 
 </Intro>