Ngrx / Redux Gotcha: Don't Forget To Return "State" In The Reducer Function

12/25/2016

Here's an error I was having for the past few days that I'm a little embarrassed to admit. haha. As it t urns out the error was caused by me not returning anthing from a certain case of the swtich statement in my reducer function.

The Problem

The problem all began when I started adding new actions to my project. I initially just added the actions to the switch statement inside of my reducer function, but out of habit I just ended the case statement with a "break". For a case inside of a reducer function though we don't want to just end the loop and return nothing because remember, the output of your reducer is the thing that will be the new state. If you just end with a break and return undefined... then the entire state becomes undefined! And that's exactly what was happening to me. Here's an example of some bad code:

export const mainStoreReducer: ActionReducer<State> =
  (state = intitialState, action: Action) => {
    switch (action.type) {

      case ActionTypes.AUTH_SUCCESS: {
        // TODO Implement reducer function for this action.

         break;
      }
  }

And here's the error it was causing in my debug console:

zone.js:388Unhandled Promise rejection: Error in ./MainComponent class MainComponent - inline template:2:4 caused by: Cannot read property 'userData' of undefined ; Zone: angular ; Task: Promise.then ; Value: ViewWrappedError {_nativeError: Error: Error in ./MainComponent class MainComponent - inline template:2:4 caused by: Cannot read pro…, originalError: TypeError: Cannot read property 'userData' of undefined
at MapSubscriber.project (http://localho…, context: DebugContext}

The important bit in the output above is the part that says, "Cannot read property 'userData' of undefined". It was telling me that this error was occurring here where I was selecting this 'userData' property:

this.store.select(s => s.mainStoreReducer.userData)
  .subscribe((data: any) => {
  console.log('data is' + data)
});

The Solution

The solution, of course, it to always return a state object (and because this is TypeScript we can say we must return some object that implements the interface defined by your state). Since the current state is passed in as an agument to the reducer function we could always just return that if we don't want our reducer to do anything:

case ActionTypes.AUTH_SUCCESS: {

   // TODO Implement reducer function for this action.
   
   return state;
}

Once you start implementing the logic for this action you'll probably want to make a clone of the current state, modify some property on the new clone, and then return the clone to be set as the new state. Well the Object.assign() method does just that (but be sure not to forget the empty object in the arguments).

return Object.assign({}, state,  {
    userData: {
    counter:action.payload.counter + 1}
  }
)

The Lesson

The life lession here for you to take home and tell to your grandkids is that inside of a Redux reducer function you HAVE to return something that implements your state. Do not use a break statement. Return a modified copy of the state. Sleep tight new york.✌

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