cRX Explained: What It Is and How It Works

Implementing cRX: A Step-by-Step Guide for Developers—

What is cRX?

cRX is a modular framework designed to simplify reactive state management and asynchronous data flows in modern applications. It combines ideas from reactive programming, event sourcing, and unidirectional data flow to provide a predictable, testable, and scalable architecture for front-end and back-end systems alike.

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Why use cRX?

• Predictability: Clear flow of data and side effects.
• Testability: Isolated units of logic make unit and integration testing straightforward.
• Scalability: Modular structure enables growth without entangling concerns.
• Performance: Fine-grained reactivity minimizes unnecessary computations and re-renders.

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Core concepts

• Store: Holds state; exposes a read interface and a controlled write interface.
• Actions/Events: Descriptions of user intents or external inputs.
• Reducers/Mutators: Pure functions that describe state changes based on actions.
• Effects/Side Effects: Managed asynchronous operations (network, timers, IO).
• Selectors: Derived data computed from the store for efficient consumption.
• Middleware/Interceptors: Pluggable logic that can observe, transform, or short-circuit actions.

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Project setup

1. Choose platform (web, mobile, server).
2. Create project skeleton (e.g., using create-react-app, Vite, Next.js, or Node + TypeScript).
3. Install dependencies (cRX core library, if available, plus helpers for TypeScript, testing, and HTTP). Example (npm):

   
   npm init -y npm install crx-core crx-effects npm install -D typescript jest ts-jest @types/jest 

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Step 1 — Define state shape

Design a clear, minimal state model. Example for a todo app:

type Todo = { id: string; title: string; completed: boolean; createdAt: string; } type TodosState = {   byId: Record<string, Todo>;   allIds: string[];   loading: boolean;   error?: string | null; } 

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Step 2 — Define actions/events

Keep actions small and intention-revealing:

type Action =   | { type: 'LOAD_TODOS' }   | { type: 'LOAD_TODOS_SUCCESS'; payload: Todo[] }   | { type: 'LOAD_TODOS_FAILURE'; error: string }   | { type: 'ADD_TODO'; payload: { title: string } }   | { type: 'TOGGLE_TODO'; payload: { id: string } } 

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Step 3 — Implement reducers/mutators

Reducers should be pure and predictable:

function todosReducer(state: TodosState, action: Action): TodosState {   switch (action.type) {     case 'LOAD_TODOS':       return { ...state, loading: true, error: null };     case 'LOAD_TODOS_SUCCESS': {       const byId = Object.fromEntries(action.payload.map(t => [t.id, t]));       return { ...state, loading: false, byId, allIds: action.payload.map(t => t.id) };     }     case 'ADD_TODO': {       const id = generateId();       const todo: Todo = { id, title: action.payload.title, completed: false, createdAt: new Date().toISOString() };       return { ...state, byId: { ...state.byId, [id]: todo }, allIds: [...state.allIds, id] };     }     case 'TOGGLE_TODO': {       const id = action.payload.id;       const item = state.byId[id];       if (!item) return state;       return { ...state, byId: { ...state.byId, [id]: { ...item, completed: !item.completed } } };     }     case 'LOAD_TODOS_FAILURE':       return { ...state, loading: false, error: action.error };     default:       return state;   } } 

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Step 4 — Implement effects (side effects)

Use effects to handle async operations and bridge to external APIs:

async function loadTodosEffect(dispatch: (a: Action) => void) {   dispatch({ type: 'LOAD_TODOS' });   try {     const res = await fetch('/api/todos');     const todos: Todo[] = await res.json();     dispatch({ type: 'LOAD_TODOS_SUCCESS', payload: todos });   } catch (err) {     dispatch({ type: 'LOAD_TODOS_FAILURE', error: String(err) });   } } 

Effect orchestration patterns: single-shot on startup, triggered by actions, or polling/scheduled.

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Step 5 — Create selectors

Selectors compute derived data and help memoize heavy calculations:

const selectAllTodos = (state: TodosState) => state.allIds.map(id => state.byId[id]); const selectActiveCount = (state: TodosState) => selectAllTodos(state).filter(t => !t.completed).length; 

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Step 6 — Compose the store

Wire reducers, effects, and middleware into a store with a clear API:

interface Store<S> {   getState(): S;   dispatch(action: Action): void;   subscribe(listener: () => void): () => void; } function createStore(initial: TodosState): Store<TodosState> {   let state = initial;   const listeners = new Set<() => void>();   return {     getState: () => state,     dispatch(action) {       state = todosReducer(state, action);       listeners.forEach(l => l());       // run effects that are triggered by action     },     subscribe(l) { listeners.add(l); return () => listeners.delete(l); }   }; } 

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Step 7 — Middleware and dev ergonomics

Add middleware for logging, crash reporting, time-travel debugging, and batching. Example logger:

const logger = (store: Store<TodosState>) => (next: (a: Action) => void) => (action: Action) => {   console.groupCollapsed(action.type);   console.log('prev', store.getState());   console.log('action', action);   next(action);   console.log('next', store.getState());   console.groupEnd(); }; 

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Step 8 — Testing

• Unit-test reducers (pure functions) with various action sequences.
• Mock effects and assert dispatched actions.
• Integration tests for store + effects (use fake timers and network stubs).
  Example Jest test for reducer:

  
  test('add todo', () => { const start: TodosState = { byId: {}, allIds: [], loading: false }; const next = todosReducer(start, { type: 'ADD_TODO', payload: { title: 'Hi' } }); expect(next.allIds.length).toBe(1); }); 

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Step 9 — Performance tuning

• Use memoized selectors (e.g., reselect) for expensive derived data.
• Batch updates to reduce re-renders.
• Normalize large lists and use windowing for long lists.
• Use web workers for CPU-heavy transforms.

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Step 10 — Deployment and observability

• Expose metrics: action throughput, effect latencies, error rates.
• Add structured logging around critical effects.
• Graceful migration: version your state and provide migration functions.

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Example: cRX in a React app (brief)

• Create store and provide with Context.
• Use custom hooks useCRXSelector and useCRXDispatch to access state and dispatch actions.
• Keep components thin: trigger actions and render selector outputs.

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Common pitfalls and how to avoid them

• Putting side effects in reducers — keep reducers pure.
• Over-normalization that makes access awkward — balance denormalization for UI needs.
• Overusing global state — prefer local component state when appropriate.
• Not versioning state — add migrations for breaking changes.

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Further reading and resources

• Reactive programming patterns
• Unidirectional data flow case studies
• Testing async code and effects

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Implementing cRX is mainly about clear boundaries: pure reducers for state transitions, controlled effects for async, and small, intention-revealing actions. Follow the steps above to build predictable, testable systems that scale.