Enforce reducer type in handleAction (#156)

README.md

@@ -132,17 +132,17 @@ expect(actionThree(3)).to.deep.equal({
 });
 ```
 
-### `handleAction(type, reducer | reducerMap, defaultState)`
+### `handleAction(type, reducer | reducerMap = Identity, defaultState)`
 
 ```js
 import { handleAction } from 'redux-actions';
 ```
 
 Wraps a reducer so that it only handles Flux Standard Actions of a certain type.
 
-If a single reducer is passed, it is used to handle both normal actions and failed actions. (A failed action is analogous to a rejected promise.) You can use this form if you know a certain type of action will never fail, like the increment example above.
+If a `reducer` function is passed, it is used to handle both normal actions and failed actions. (A failed action is analogous to a rejected promise.) You can use this form if you know a certain type of action will never fail, like the increment example above.
 
-Otherwise, you can specify separate reducers for `next()` and `throw()`. This API is inspired by the ES6 generator interface.
+Otherwise, you can specify separate reducers for `next()` and `throw()` using the `reducerMap` form. This API is inspired by the ES6 generator interface.
 
 ```js
 handleAction('FETCH_DATA', {
@@ -151,7 +151,9 @@ handleAction('FETCH_DATA', {
 }, defaultState);
 ```
 
-If either `next()` or `throw()` are `undefined` or `null`, then the identity function is used for that reducer.
+If either `next()` or `throw()` are `undefined` or `null`, then the identity function is used for that reducer. 
+
+If the reducer argument (`reducer | reducerMap`) is `undefined`, then the identity function is used.
 
 The third parameter `defaultState` is required, and is used when `undefined` is passed to the reducer.
 

src/__tests__/handleAction-test.js

@@ -7,6 +7,26 @@ describe('handleAction()', () => {
   const prevState = { counter: 3 };
   const defaultState = { counter: 0 };
 
+  it('should throw an error if the reducer is the wrong type', () => {
+    const wrongTypeReducers = [1, 'string', [], null];
+
+    wrongTypeReducers.forEach(wrongTypeReducer => {
+      expect(() => {
+        handleAction(type, wrongTypeReducer, defaultState);
+      }).to.throw(
+        Error,
+        'Expected reducer to be a function or object with next and throw reducers'
+      );
+    });
+  });
+
+  it('uses the identity if the specified reducer is undefined', () => {
+    const reducer = handleAction(type, undefined, defaultState);
+
+    expect(reducer(prevState, { type })).to.equal(prevState);
+    expect(reducer(prevState, { type, error: true, payload: new Error })).to.equal(prevState);
+  });
+
   describe('single handler form', () => {
     it('should throw an error if defaultState is not specified', () => {
       expect(() => {

src/handleAction.js

@@ -1,4 +1,5 @@
 import isFunction from 'lodash/isFunction';
+import isPlainObject from 'lodash/isPlainObject';
 import identity from 'lodash/identity';
 import isNil from 'lodash/isNil';
 import isUndefined from 'lodash/isUndefined';
@@ -7,16 +8,20 @@ import invariant from 'invariant';
 import { isFSA } from 'flux-standard-action';
 import { ACTION_TYPE_DELIMITER } from './combineActions';
 
-export default function handleAction(actionType, reducers, defaultState) {
+export default function handleAction(actionType, reducer = identity, defaultState) {
   const actionTypes = actionType.toString().split(ACTION_TYPE_DELIMITER);
   invariant(
     !isUndefined(defaultState),
     `defaultState for reducer handling ${actionTypes.join(', ')} should be defined`
   );
+  invariant(
+    isFunction(reducer) || isPlainObject(reducer),
+    'Expected reducer to be a function or object with next and throw reducers'
+  );
 
-  const [nextReducer, throwReducer] = isFunction(reducers)
-    ? [reducers, reducers]
-    : [reducers.next, reducers.throw].map(reducer => (isNil(reducer) ? identity : reducer));
+  const [nextReducer, throwReducer] = isFunction(reducer)
+    ? [reducer, reducer]
+    : [reducer.next, reducer.throw].map(aReducer => (isNil(aReducer) ? identity : aReducer));
 
   return (state = defaultState, action) => {
     invariant(