Redux Toolkit Patterns

Quick Guide: Use Redux Toolkit for complex client state requiring DevTools, middleware, or entity normalization. Use RTK Query for data fetching with caching. For simpler UI state, a lighter state management solution may be more appropriate. NEVER use legacy Redux patterns (createStore, combineReducers manually, switch statements in reducers).

Detailed Resources:

• For code examples, see examples/ folder:
  • core.md - Store setup, slices
  • typed-hooks.md - Type-safe hooks
  • rtk-query.md - Data fetching
  • entity-adapters.md - Normalized state
  • async-thunks.md - Async operations
  • selectors.md - Memoized selectors
  • rtk-2-features.md - RTK 2.0: combineSlices, inline selectors, buildCreateSlice
  • middleware.md - Custom middleware
  • testing.md - Testing patterns
  • integrations.md - Redux Persist
• For decision frameworks and anti-patterns, see reference.md

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<critical_requirements>

CRITICAL: Before Managing State with Redux Toolkit

(You MUST use configureStore for store setup - NEVER legacy createStore)

(You MUST use createSlice for all reducers - NEVER switch statements or manual action creators)

(You MUST define typed hooks (useAppSelector, useAppDispatch) once in a hooks file)

(You MUST use named exports ONLY - NO default exports in any Redux files)

(You MUST use named constants for ALL numbers - NO magic numbers in state code)

</critical_requirements>

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Auto-detection: Redux Toolkit, createSlice, configureStore, RTK Query, createAsyncThunk, createEntityAdapter, useSelector, useDispatch

When to use:

• Complex client state requiring middleware, DevTools, or time-travel debugging
• Enterprise applications with multiple teams needing predictable state management
• Normalized entity state (lists of items with relationships)
• Data fetching with sophisticated caching (RTK Query)
• Applications requiring strict unidirectional data flow

Key patterns covered:

• Store configuration with configureStore
• Slice creation with createSlice and Immer integration
• RTK 2.0: Inline selectors in createSlice, combineSlices, buildCreateSlice for async thunks
• RTK Query for data fetching and caching
• Typed hooks for TypeScript integration
• Entity adapters for normalized state
• Async thunks with createAsyncThunk
• Middleware patterns

When NOT to use:

• Simple UI state (useState or a lightweight state management solution)
• Server state only (use a dedicated data fetching solution)
• Small to medium apps where Redux overhead is unnecessary
• Projects where team unfamiliarity with Redux outweighs benefits

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Philosophy

Redux Toolkit (RTK) is the official, opinionated, batteries-included toolset for efficient Redux development. It eliminates the boilerplate of legacy Redux while maintaining predictable state management through strict unidirectional data flow.

Core principle: "One source of truth" - All application state lives in a single store, changes are made through pure reducer functions, and state is never mutated directly.

RTK uses Immer internally, allowing you to write "mutative" code that is actually immutable. This dramatically simplifies reducer logic while maintaining Redux's guarantees.

Key architecture decisions:

1. configureStore replaces createStore with sensible defaults (DevTools, thunk middleware, development checks)
2. createSlice generates action creators and action types automatically from reducer names
3. RTK Query provides a purpose-built data fetching and caching solution
4. createEntityAdapter standardizes normalized state patterns

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Core Patterns

Pattern 1: Store Configuration

Use configureStore with proper TypeScript types inferred from the store itself.

For store setup, typed hooks, and middleware configuration examples, see examples/core.md.

---

Pattern 2: Slice Creation with createSlice

Use createSlice to define reducers, actions, and initial state together. Immer allows "mutative" syntax for immutable updates.

When to Use

• All Redux state management
• Generating action creators automatically
• Simplifying immutable update logic

When NOT to Use

• State that belongs in URL params (filters, search)
• Truly local component state (useState)
• Server state that should use RTK Query or a data fetching solution

For slice creation with typed state and PayloadAction, see examples/core.md.

---

Pattern 3: Typed Hooks for Components

Define typed versions of useDispatch and useSelector once, then use everywhere. This ensures type safety without repetitive type annotations.

Why Typed Hooks Matter

• useSelector saves typing (state: RootState) every time
• useDispatch default type does not know about thunks
• AppDispatch includes thunk middleware types for correct dispatch typing

For typed hooks setup with .withTypes() (React Redux v9.1.0+), see examples/typed-hooks.md.

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Pattern 4: RTK Query for Data Fetching

RTK Query is a purpose-built data fetching and caching solution. Use it when you need caching, automatic refetching, and cache invalidation.

When to Use RTK Query

• Data fetching with caching requirements
• Automatic refetch on focus/reconnect
• Cache invalidation with tags
• Optimistic updates for mutations

When NOT to Use

• Simple client state (use createSlice)
• When an existing data fetching solution is already established in the codebase
• When you need features RTK Query does not support

For createApi setup, endpoint definitions, and cache tags, see examples/rtk-query.md.

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Pattern 5: Entity Adapters for Normalized State

Use createEntityAdapter for collections of items (users, products, posts). It provides standardized CRUD operations and memoized selectors.

When to Use

• Lists of items with IDs
• Relational data that needs normalization
• CRUD operations on collections
• Performance-critical large lists

For entity adapter setup, CRUD operations, and selector usage, see examples/entity-adapters.md.

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Pattern 6: Async Thunks with createAsyncThunk

Use createAsyncThunk for async operations that need to dispatch multiple actions (pending, fulfilled, rejected).

When to Use

• Complex async flows not suited for RTK Query
• Operations that need access to state during async flow
• Chained async operations
• Legacy code migration

When NOT to Use

• Standard API calls (prefer RTK Query)
• Simple sync state updates (use createSlice actions)

For async thunk creation and handling lifecycle actions, see examples/async-thunks.md.

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Pattern 7: Selectors and Memoization

Create reusable selectors for derived state. Use createSelector from Reselect for memoized computed values.

For selector patterns and memoization, see examples/selectors.md.

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Pattern 8: Middleware Patterns

Add custom middleware for logging, analytics, or side effects. Use the middleware callback to extend default middleware.

For middleware configuration and custom middleware, see examples/middleware.md.

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<red_flags>

RED FLAGS

High Priority Issues:

• Using legacy createStore instead of configureStore -- misses DevTools, development checks, and middleware defaults
• Switch statements in reducers -- use createSlice which generates action creators automatically
• Manual action type strings -- createSlice generates these from reducer names
• Mutating state outside Immer context -- only "mutate" inside createSlice/createReducer
• Not adding RTK Query middleware to store -- caching, polling, and invalidation silently fail
• RTK Query cache not blacklisted in redux-persist -- causes stale cache restoration on rehydration
• RTK 2.0: Object syntax in extraReducers -- removed in v2, use builder callback
• RTK 2.0: AnyAction type deprecated -- use UnknownAction with isAction() guard

Medium Priority Issues:

• Using untyped useDispatch/useSelector instead of typed hooks -- loses type safety for thunks and state
• Defining typed hooks in the store file -- causes circular imports; put in separate hooks.ts
• Storing derived state in the store -- compute in selectors instead
• Not calling setupListeners for RTK Query -- refetchOnFocus/refetchOnReconnect will not work
• Not using rejectWithValue in thunks -- loses typed error handling
• Not using createSelector for derived data -- causes unnecessary recalculations on every render

Gotchas & Edge Cases:

• Immer mutations only work inside createSlice/createReducer -- not in action creators or thunks
• Entity adapter updateOne/updateMany perform shallow merge -- deep nested updates need manual handling
• RTK Query cache tags are case-sensitive -- "User" !== "user"
• createAsyncThunk auto-dispatches pending/fulfilled/rejected -- do not dispatch these manually
• getDefaultMiddleware() must be called (not referenced) in middleware config
• TypeScript infers RootState from store.getState return type -- keep slice state types accurate

</red_flags>

See reference.md for decision frameworks, anti-patterns with code examples, migration guides, and performance considerations.

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<critical_reminders>

CRITICAL REMINDERS

(You MUST use configureStore for store setup - NEVER legacy createStore)

(You MUST use createSlice for all reducers - NEVER switch statements or manual action creators)

(You MUST define typed hooks (useAppSelector, useAppDispatch) once in a hooks file)

(You MUST use named exports ONLY - NO default exports in any Redux files)

(You MUST use named constants for ALL numbers - NO magic numbers in state code)

Failure to follow these rules will cause type safety issues, unnecessary boilerplate, and convention violations.

</critical_reminders>