React useReducer Hook

1. useReducer Syntax and Dispatch Patterns (React.useReducer)

Component	Syntax	Description	Usage
useReducer Hook	`useReducer(reducer, initialState)`	Returns `[state, dispatch]` array	Alternative to useState for complex state logic
With Init Function	`useReducer(reducer, initialArg, init)`	Third argument initializes state lazily	Lazy initialization: `init(initialArg)` called once
Dispatch Function	`dispatch(action)`	Triggers state update via reducer function	Stable reference - safe for useEffect dependencies

Dispatch Pattern	Syntax	Action Structure	Use Case
Type Only	`dispatch({ type: 'INCREMENT' })`	Action with type, no payload	Simple state transitions without data
Type + Payload	`dispatch({ type: 'SET_VALUE', payload: value })`	Action with type and data	Most common - pass data to reducer
Spread Payload	`dispatch({ type: 'UPDATE', ...data })`	Flatten payload into action object	Multiple values: `{ type, id, name, age }`
FSA (Flux Standard Action)	`dispatch({ type, payload, error, meta })`	Standardized action shape	Industry standard for Redux-style actions

Example: Basic useReducer setup and dispatch

import { useReducer } from 'react';

// Reducer function
function counterReducer(state, action) {
  switch (action.type) {
    case 'INCREMENT':
      return { count: state.count + 1 };
    case 'DECREMENT':
      return { count: state.count - 1 };
    case 'SET':
      return { count: action.payload };
    case 'RESET':
      return { count: 0 };
    default:
      throw new Error(`Unknown action: ${action.type}`);
  }
}

function Counter() {
  const [state, dispatch] = useReducer(counterReducer, { count: 0 });
  
  return (
    <div>
      <p>Count: {state.count}</p>
      <button onClick={() => dispatch({ type: 'INCREMENT' })}>+</button>
      <button onClick={() => dispatch({ type: 'DECREMENT' })}>-</button>
      <button onClick={() => dispatch({ type: 'SET', payload: 10 })}>Set 10</button>
      <button onClick={() => dispatch({ type: 'RESET' })}>Reset</button>
    </div>
  );
}

Example: Lazy initialization with init function

// Init function for lazy initialization
function init(initialCount) {
  return { count: initialCount, history: [] };
}

function CounterWithHistory({ initialCount }) {
  // Third argument: init function called once with initialCount
  const [state, dispatch] = useReducer(reducer, initialCount, init);
  
  // state = { count: initialCount, history: [] }
}

2. Reducer Function Patterns and Action Types

Reducer Pattern	Implementation	Pros	Cons
Switch Statement	`switch(action.type) { case 'TYPE': return newState; }`	Clear, explicit, easy to read	Verbose for many action types
Object Lookup	`const handlers = {TYPE: (state) => newState}; return handlers[action.type]?.(state)`	Concise, functional style	Less explicit error handling
If-Else Chain	`if (type === 'A') return ...; else if (type === 'B') return ...;`	Simple for few actions	Poor scalability, hard to maintain

Action Type Convention	Format	Example	Use Case
UPPER_SNAKE_CASE	`'ACTION_NAME'`	`'FETCH_USER', 'UPDATE_PROFILE'`	Traditional Redux style, clear constants
Namespaced	`'domain/ACTION'`	`'user/FETCH', 'cart/ADD_ITEM'`	Organize by feature, prevent collisions
Enum/Constants	`const ACTIONS = {ADD: 'ADD'}`	`ACTIONS.ADD, ACTIONS.REMOVE`	Type safety, autocomplete, refactoring

Example: Reducer patterns comparison

// Pattern 1: Switch statement (most common)
function reducer(state, action) {
  switch (action.type) {
    case 'ADD_ITEM':
      return { ...state, items: [...state.items, action.payload] };
    case 'REMOVE_ITEM':
      return { ...state, items: state.items.filter(i => i.id !== action.payload) };
    case 'CLEAR':
      return { ...state, items: [] };
    default:
      return state; // Or throw error for unknown actions
  }
}

// Pattern 2: Object lookup (functional style)
const handlers = {
  ADD_ITEM: (state, action) => ({
    ...state,
    items: [...state.items, action.payload]
  }),
  REMOVE_ITEM: (state, action) => ({
    ...state,
    items: state.items.filter(i => i.id !== action.payload)
  }),
  CLEAR: (state) => ({ ...state, items: [] })
};

function reducer(state, action) {
  const handler = handlers[action.type];
  return handler ? handler(state, action) : state;
}

// Pattern 3: Action type constants (type safety)
const ACTIONS = {
  ADD_ITEM: 'ADD_ITEM',
  REMOVE_ITEM: 'REMOVE_ITEM',
  CLEAR: 'CLEAR'
} as const;

function reducer(state, action) {
  switch (action.type) {
    case ACTIONS.ADD_ITEM:
      return { ...state, items: [...state.items, action.payload] };
    case ACTIONS.REMOVE_ITEM:
      return { ...state, items: state.items.filter(i => i.id !== action.payload) };
    case ACTIONS.CLEAR:
      return { ...state, items: [] };
    default:
      return state;
  }
}

Best Practice: Always return a new state object. Never mutate the existing state. Use spread operators or Immer for immutable updates.

3. Action Creators and Payload Structures

Pattern	Implementation	Benefit
Inline Action	`dispatch({ type: 'ADD', payload: item })`	Simple, direct - good for one-off actions
Action Creator RECOMMENDED	`const addItem = (item) => ({ type: 'ADD', payload: item })`	Reusable, testable, consistent structure
Typed Action Creator	`const addItem = (item: Item): Action => ({ type: 'ADD', payload: item })`	Type safety with TypeScript

Payload Structure	Example	Use Case
Single Value	`{ type: 'SET_NAME', payload: 'John' }`	Simple value updates
Object Payload	`{ type: 'UPDATE_USER', payload: { name, age, email } }`	Multiple related values
Indexed Payload	`{ type: 'UPDATE_ITEM', payload: { id: 1, changes: {...} } }`	Update specific item in collection
Normalized Payload	`{ type: 'ADD_MANY', payload: { byId: {...}, allIds: [...] } }`	Normalized data structures

Example: Action creators for cleaner code

// Action type constants
const ACTIONS = {
  ADD_TODO: 'ADD_TODO',
  TOGGLE_TODO: 'TOGGLE_TODO',
  DELETE_TODO: 'DELETE_TODO',
  SET_FILTER: 'SET_FILTER'
};

// Action creators
const addTodo = (text) => ({
  type: ACTIONS.ADD_TODO,
  payload: { id: Date.now(), text, completed: false }
});

const toggleTodo = (id) => ({
  type: ACTIONS.TOGGLE_TODO,
  payload: id
});

const deleteTodo = (id) => ({
  type: ACTIONS.DELETE_TODO,
  payload: id
});

const setFilter = (filter) => ({
  type: ACTIONS.SET_FILTER,
  payload: filter
});

// Usage in component
function TodoApp() {
  const [state, dispatch] = useReducer(todoReducer, initialState);
  
  const handleAdd = (text) => {
    dispatch(addTodo(text)); // Clean, readable
  };
  
  const handleToggle = (id) => {
    dispatch(toggleTodo(id)); // Clear intent
  };
  
  return (
    <div>
      <button onClick={() => handleAdd('New task')}>Add</button>
      <button onClick={() => handleToggle(1)}>Toggle</button>
    </div>
  );
}

Example: TypeScript action creators with discriminated unions

// Define action types with discriminated union
type Action =
  | { type: 'ADD_TODO'; payload: { text: string } }
  | { type: 'TOGGLE_TODO'; payload: number }
  | { type: 'DELETE_TODO'; payload: number }
  | { type: 'SET_FILTER'; payload: 'all' | 'active' | 'completed' };

// Type-safe action creators
const actions = {
  addTodo: (text: string): Action => ({
    type: 'ADD_TODO',
    payload: { text }
  }),
  toggleTodo: (id: number): Action => ({
    type: 'TOGGLE_TODO',
    payload: id
  }),
  deleteTodo: (id: number): Action => ({
    type: 'DELETE_TODO',
    payload: id
  }),
  setFilter: (filter: 'all' | 'active' | 'completed'): Action => ({
    type: 'SET_FILTER',
    payload: filter
  })
};

// Reducer with full type inference
function reducer(state: State, action: Action): State {
  switch (action.type) {
    case 'ADD_TODO':
      // TypeScript knows action.payload is { text: string }
      return { ...state, todos: [...state.todos, { id: Date.now(), ...action.payload, completed: false }] };
    case 'TOGGLE_TODO':
      // TypeScript knows action.payload is number
      return { ...state, todos: state.todos.map(t => t.id === action.payload ? { ...t, completed: !t.completed } : t) };
    default:
      return state;
  }
}

4. State Machine Patterns with useReducer

State Machine Concept	Description	Benefit
Finite States	Limited set of possible states (idle, loading, success, error)	Prevents impossible states, clearer logic
Transitions	Explicit rules for moving between states	Predictable state changes, easier debugging
Guards	Conditions that must be met for transitions	Validation logic, prevent invalid transitions

State Machine Pattern	State Structure	Use Case
Status Enum	`{ status: 'idle' \| 'loading' \| 'success' \| 'error', data?, error? }`	Async operations, API calls
Tagged Union	`{ type: 'loading' } \| { type: 'success', data } \| { type: 'error', error }`	Type-safe states with TypeScript
Nested States	`{ phase: 'editing', editMode: { step: 1 \| 2 \| 3 } }`	Multi-step processes, wizards

Example: State machine for async data fetching

// State type with discriminated union
type State =
  | { status: 'idle' }
  | { status: 'loading' }
  | { status: 'success'; data: User[] }
  | { status: 'error'; error: string };

type Action =
  | { type: 'FETCH_START' }
  | { type: 'FETCH_SUCCESS'; payload: User[] }
  | { type: 'FETCH_ERROR'; payload: string }
  | { type: 'RESET' };

function reducer(state: State, action: Action): State {
  switch (state.status) {
    case 'idle':
      if (action.type === 'FETCH_START') {
        return { status: 'loading' };
      }
      break;
    
    case 'loading':
      if (action.type === 'FETCH_SUCCESS') {
        return { status: 'success', data: action.payload };
      }
      if (action.type === 'FETCH_ERROR') {
        return { status: 'error', error: action.payload };
      }
      break;
    
    case 'success':
    case 'error':
      if (action.type === 'FETCH_START') {
        return { status: 'loading' };
      }
      if (action.type === 'RESET') {
        return { status: 'idle' };
      }
      break;
  }
  
  return state; // No valid transition
}

function UserList() {
  const [state, dispatch] = useReducer(reducer, { status: 'idle' });
  
  const fetchUsers = async () => {
    dispatch({ type: 'FETCH_START' });
    try {
      const data = await fetch('/api/users').then(r => r.json());
      dispatch({ type: 'FETCH_SUCCESS', payload: data });
    } catch (error) {
      dispatch({ type: 'FETCH_ERROR', payload: error.message });
    }
  };
  
  if (state.status === 'loading') return <div>Loading...</div>;
  if (state.status === 'error') return <div>Error: {state.error}</div>;
  if (state.status === 'success') return <div>{state.data.length} users</div>;
  return <button onClick={fetchUsers}>Load Users</button>;
}

Impossible States: State machines prevent scenarios like { loading: true, error: 'Failed', data: [...] } which are logically impossible but possible with boolean flags.

5. useReducer vs useState - When to Use Which

Criterion	Use useState	Use useReducer
State Complexity	Simple primitives or shallow objects	Complex nested objects, multiple related values
Update Logic	Simple assignments: `setState(value)`	Complex logic, conditionals, multiple steps
Number of Updates	Few state update locations	Many different update operations
Predictability	Simple, local state changes	Need centralized, traceable state transitions
Testing	Simple component tests sufficient	Want to test reducer logic in isolation
Future Changes	State unlikely to grow in complexity	Anticipate adding more actions/transitions

Scenario	Best Choice	Reason
Toggle boolean flag	✅ useState	Simple on/off state, no complex logic
Form with validation	✅ useReducer	Multiple fields, validation rules, error states
Shopping cart	✅ useReducer	Add/remove/update/clear operations, quantities
Modal open/close	✅ useState	Binary state, simple toggle
Multi-step wizard	✅ useReducer	State machine with steps, navigation, data
Counter	✅ useState	Single value, simple increment/decrement
Async data fetching	✅ useReducer	Loading/success/error states, state machine

Example: When to migrate from useState to useReducer

// ❌ useState becomes unwieldy with complex state
function ComplexForm() {
  const [name, setName] = useState('');
  const [email, setEmail] = useState('');
  const [age, setAge] = useState(0);
  const [errors, setErrors] = useState({});
  const [touched, setTouched] = useState({});
  const [isSubmitting, setIsSubmitting] = useState(false);
  const [submitError, setSubmitError] = useState(null);
  
  // Hard to coordinate updates across multiple setState calls
  const handleSubmit = async () => {
    setIsSubmitting(true);
    setSubmitError(null);
    // ... complex logic
  };
}

// ✅ useReducer centralizes complex state management
function ComplexForm() {
  const [state, dispatch] = useReducer(formReducer, {
    values: { name: '', email: '', age: 0 },
    errors: {},
    touched: {},
    isSubmitting: false,
    submitError: null
  });
  
  // Single dispatch for coordinated updates
  const handleSubmit = async () => {
    dispatch({ type: 'SUBMIT_START' });
    try {
      await submitForm(state.values);
      dispatch({ type: 'SUBMIT_SUCCESS' });
    } catch (error) {
      dispatch({ type: 'SUBMIT_ERROR', payload: error.message });
    }
  };
}

Migration Checklist: useState → useReducer

State has 3+ related pieces of data Multiple setState calls often happen together State update logic has complex conditionals Need to test state logic independently State transitions follow predictable patterns Team prefers Redux-style architecture

6. Async Actions and useReducer Integration

Pattern	Implementation	Use Case
Dispatch in Async Handler	Call dispatch before/after async operations in event handlers	Simple async flows in components
Thunk Pattern	Function that receives dispatch and executes async logic	Reusable async operations, middleware-like
useEffect Integration	useEffect with dispatch for side effects	Fetch on mount, sync with external systems
Custom Hook	Wrap useReducer + async logic in custom hook	Encapsulate complex async state management

Async State Pattern	State Structure	Actions
Loading/Error/Data	`{ loading: boolean, error: Error?, data: T? }`	START, SUCCESS, ERROR
Status Enum	`{ status: 'idle' \| 'loading' \| 'success' \| 'error', ... }`	FETCH_START, FETCH_SUCCESS, FETCH_ERROR, RESET
Request Tracking	`{ requestId: string, pending: boolean, ... }`	Track request IDs to prevent race conditions

Example: Async actions with useReducer

type State = {
  data: User[] | null;
  loading: boolean;
  error: string | null;
};

type Action =
  | { type: 'FETCH_START' }
  | { type: 'FETCH_SUCCESS'; payload: User[] }
  | { type: 'FETCH_ERROR'; payload: string };

function reducer(state: State, action: Action): State {
  switch (action.type) {
    case 'FETCH_START':
      return { data: null, loading: true, error: null };
    case 'FETCH_SUCCESS':
      return { data: action.payload, loading: false, error: null };
    case 'FETCH_ERROR':
      return { data: null, loading: false, error: action.payload };
    default:
      return state;
  }
}

function UserList() {
  const [state, dispatch] = useReducer(reducer, {
    data: null,
    loading: false,
    error: null
  });
  
  // Async handler with dispatch
  const fetchUsers = async () => {
    dispatch({ type: 'FETCH_START' });
    
    try {
      const response = await fetch('/api/users');
      const data = await response.json();
      dispatch({ type: 'FETCH_SUCCESS', payload: data });
    } catch (error) {
      dispatch({ type: 'FETCH_ERROR', payload: error.message });
    }
  };
  
  // Fetch on mount
  useEffect(() => {
    fetchUsers();
  }, []);
  
  if (state.loading) return <div>Loading...</div>;
  if (state.error) return <div>Error: {state.error}</div>;
  if (state.data) return <ul>{state.data.map(u => <li key={u.id}>{u.name}</li>)}</ul>;
  return null;
}

Example: Custom async hook with useReducer

// Reusable async hook
function useAsync<T>(asyncFunction: () => Promise<T>) {
  const [state, dispatch] = useReducer(
    (state: AsyncState<T>, action: AsyncAction<T>) => {
      switch (action.type) {
        case 'LOADING': return { status: 'loading' };
        case 'SUCCESS': return { status: 'success', data: action.payload };
        case 'ERROR': return { status: 'error', error: action.payload };
        default: return state;
      }
    },
    { status: 'idle' }
  );
  
  const execute = useCallback(async () => {
    dispatch({ type: 'LOADING' });
    try {
      const data = await asyncFunction();
      dispatch({ type: 'SUCCESS', payload: data });
    } catch (error) {
      dispatch({ type: 'ERROR', payload: error.message });
    }
  }, [asyncFunction]);
  
  return { ...state, execute };
}

// Usage
function UserProfile({ userId }) {
  const { status, data, error, execute } = useAsync(() =>
    fetch(`/api/users/${userId}`).then(r => r.json())
  );
  
  useEffect(() => {
    execute();
  }, [userId, execute]);
  
  if (status === 'loading') return <div>Loading...</div>;
  if (status === 'error') return <div>Error: {error}</div>;
  if (status === 'success') return <div>{data.name}</div>;
  return null;
}

Race Conditions: Multiple async operations can complete out of order. Use request IDs or abort controllers to cancel stale requests. React 18's useTransition can also help manage async state updates.

useReducer Best Practices Summary

Use switch statements for clear, explicit action handling
Create action creators for consistent action structure
Implement state machines for complex async flows
Choose useReducer when state has 3+ related pieces
Dispatch actions in async handlers for coordinated updates
Extract async logic into custom hooks for reusability
Use TypeScript discriminated unions for type-safe reducers