Merge pull request #1386 from reactjs/useReducer-example

Updated useReducer reducer example to show default case

Author: bvaughn

content/docs/hooks-reference.md

@@ -190,6 +190,10 @@ function reducer(state, action) {
       return {count: state.count + 1};
     case 'decrement':
       return {count: state.count - 1};
+    default:
+      // A reducer must always return a valid state.
+      // Alternatively you can throw an error if an invalid action is dispatched.
+      return state;
   }
 }
 
@@ -223,6 +227,10 @@ function reducer(state, action) {
       return {count: state.count + 1};
     case 'decrement':
       return {count: state.count - 1};
+    default:
+      // A reducer must always return a valid state.
+      // Alternatively you can throw an error if an invalid action is dispatched.
+      return state;
   }
 }