How to fix Reducer returned undefined after action, check your action type in React

function myReducer(state = initialState, action) {
  switch (action.type) {
    case 'SOME_ACTION':
      return { ...state, ...action.payload };
    default:
      return state; // Always include this default case
  }
}

// Incorrect - missing return
case 'UPDATE_DATA':
  state.data = action.data; // This doesn't return!
  break;

// Correct - returns new state
case 'UPDATE_DATA':
  return { ...state, data: action.data };

// Good - provides default initial state
function reducer(state = { count: 0 }, action) {
  // reducer logic
}

// Also acceptable - explicit check
function reducer(state, action) {
  if (state === undefined) {
    return { count: 0 };
  }
  // rest of reducer
}

import { combineReducers } from 'redux';

const rootReducer = combineReducers({
  todos: todosReducer, // Must never return undefined
  visibility: visibilityFilterReducer // Must never return undefined
});

// Each reducer must handle undefined state by returning initial state
function todosReducer(state = [], action) {
  // ...
}