What is the best way to deal with a fetch error in react redux?

Question

I have one reducer for Clients, one other for AppToolbar and some others...

Now lets say that I created a fetch action to delete client, and if it fails I have code

Answer 1

The approach I'm currently taking for a few specific errors (user input validation) is to have my sub-reducers throw an exception, catch it in my root reducer, and attach it to the action object. Then I have a redux-saga that inspects action objects for an error and update the state tree with error data in that case.

So:

function rootReducer(state, action) {
  try {
    // sub-reducer(s)
    state = someOtherReducer(state,action);
  } catch (e) {
    action.error = e;
  }
  return state;
}

// and then in the saga, registered to take every action:
function *errorHandler(action) {
  if (action.error) {
     yield put(errorActionCreator(error));
  }
}

And then adding the error to the state tree is as Erik describes.

I use it pretty sparingly, but it keeps me from having to duplicate logic which legitimately belongs in the reducer (so it can protect itself from an invalid state).