Add support for default or initial state to handleAction. (#55)

Author: seniorquico

README.md

@@ -57,7 +57,7 @@ createAction('ADD_TODO')('Use Redux');
 
 `metaCreator` is an optional function that creates metadata for the payload. It receives the same arguments as the payload creator, but its result becomes the meta field of the resulting action. If `metaCreator` is undefined or not a function, the meta field is omitted.
 
-### `handleAction(type, reducer | reducerMap)`
+### `handleAction(type, reducer | reducerMap, ?defaultState)`
 
 Wraps a reducer so that it only handles Flux Standard Actions of a certain type.
 
@@ -72,6 +72,8 @@ handleAction('FETCH_DATA', {
 });
 ```
 
+The optional third parameter specifies a default or initial state, which is used when `undefined` is passed to the reducer.
+
 ### `handleActions(reducerMap, ?defaultState)`
 
 Creates multiple reducers using `handleAction()` and combines them into a single reducer that handles multiple actions. Accepts a map where the keys are passed as the first parameter to `handleAction()` (the action type), and the values are passed as the second parameter (either a reducer or reducer map).

src/__tests__/handleAction-test.js

@@ -11,6 +11,14 @@ describe('handleAction()', () => {
         expect(reducer(prevState, { type })).to.equal(prevState);
       });
 
+      it('returns default state if type does not match', () => {
+        const reducer = handleAction('NOTTYPE', () => null, { counter: 7 });
+        expect(reducer(undefined, { type }))
+          .to.deep.equal({
+            counter: 7
+          });
+      });
+
       it('accepts single function as handler', () => {
         const reducer = handleAction(type, (state, action) => ({
           counter: state.counter + action.payload
@@ -20,6 +28,16 @@ describe('handleAction()', () => {
             counter: 10
           });
       });
+
+      it('accepts single function as handler and a default state', () => {
+        const reducer = handleAction(type, (state, action) => ({
+          counter: state.counter + action.payload
+        }), { counter: 3 });
+        expect(reducer(undefined, { type, payload: 7 }))
+          .to.deep.equal({
+            counter: 10
+          });
+      });
     });
   });
 

src/handleAction.js

@@ -2,8 +2,8 @@ function isFunction(val) {
   return typeof val === 'function';
 }
 
-export default function handleAction(type, reducers) {
-  return (state, action) => {
+export default function handleAction(type, reducers, defaultState) {
+  return (state = defaultState, action) => {
     // If action type does not match, return previous state
     if (action.type !== type) return state;