move combine reducers sanity check

Author: ellbee

src/utils/combineReducers.js

@@ -72,11 +72,12 @@ function verifyStateShape(inputState, outputState, action) {
 
 export default function combineReducers(reducers) {
   var finalReducers = pick(reducers, (val) => typeof val === 'function');
+  var sanityError;
 
   Object.keys(finalReducers).forEach(key => {
     var reducer = finalReducers[key];
-    if (typeof reducer(undefined, { type: ActionTypes.INIT }) === 'undefined') {
-      throw new Error(
+    if (!sanityError && typeof reducer(undefined, { type: ActionTypes.INIT }) === 'undefined') {
+      sanityError = new Error(
         `Reducer "${key}" returned undefined during initialization. ` +
         `If the state passed to the reducer is undefined, you must ` +
         `explicitly return the initial state. The initial state may ` +
@@ -85,8 +86,8 @@ export default function combineReducers(reducers) {
     }
 
     var type = Math.random().toString(36).substring(7).split('').join('.');
-    if (typeof reducer(undefined, { type }) === 'undefined') {
-      throw new Error(
+    if (!sanityError && typeof reducer(undefined, { type }) === 'undefined') {
+      sanityError = new Error(
         `Reducer "${key}" returned undefined when probed with a random type. ` +
         `Don't try to handle ${ActionTypes.INIT} or other actions in "redux/*" ` +
         `namespace. They are considered private. Instead, you must return the ` +
@@ -100,6 +101,9 @@ export default function combineReducers(reducers) {
   var defaultState = mapValues(finalReducers, () => undefined);
 
   return function combination(state = defaultState, action) {
+    if (sanityError) {
+      throw sanityError;
+    }
     var finalState = mapValues(finalReducers, (reducer, key) => {
       var newState = reducer(state[key], action);
       if (typeof newState === 'undefined') {

test/utils/combineReducers.spec.js

@@ -66,8 +66,8 @@ describe('Utils', () => {
       );
     });
 
-    it('should throw an error if a reducer returns undefined initializing', () => {
-      expect(() => combineReducers({
+    it('should throw an error on first call if a reducer returns undefined initializing', () => {
+      const reducer = combineReducers({
         counter(state, action) {
           switch (action.type) {
           case 'increment':
@@ -78,7 +78,8 @@ describe('Utils', () => {
             return state;
           }
         }
-      })).toThrow(
+      });
+      expect(() => reducer({})).toThrow(
         /"counter".*initialization/
       );
     });
@@ -100,8 +101,8 @@ describe('Utils', () => {
       expect(reducer({counter: 0}, { type: increment }).counter).toEqual(1);
     });
 
-    it('should throw an error if a reducer attempts to handle a private action', () => {
-      expect(() => combineReducers({
+    it('should throw an error on first call if a reducer attempts to handle a private action', () => {
+      const reducer = combineReducers({
         counter(state, action) {
           switch (action.type) {
           case 'increment':
@@ -115,7 +116,8 @@ describe('Utils', () => {
             return undefined;
           }
         }
-      })).toThrow(
+      });
+      expect(() => reducer()).toThrow(
         /"counter".*private/
       );
     });