web-state-jotai

Jotai Atomic State Management

Quick Guide: Jotai provides atomic, bottom-up state management with automatic dependency tracking. Define atoms at module level (never inside components). Use primitive atoms for values, derived atoms for computed state, write-only atoms for actions, and async atoms with Suspense for loading. Components only re-render when their specific atoms change. The atomFamily utility is deprecated -- use the jotai-family package for new code.

<critical_requirements>

CRITICAL: Before Using Jotai

(You MUST define atoms OUTSIDE components -- creating atoms inside render causes broken state)

(You MUST wrap async atom consumers in Suspense boundaries -- async atoms trigger Suspense by default)

(You MUST use write atoms (action atoms) to encapsulate multi-atom updates and post-async state changes)

(You MUST use jotai-family package instead of deprecated atomFamily from jotai/utils for new code)

</critical_requirements>

Auto-detection: Jotai, jotai, atom, useAtom, useAtomValue, useSetAtom, atomWithStorage, atomFamily, splitAtom, selectAtom, derived atom, loadable, unwrap, createStore, Provider store

When to use:

• Fine-grained reactivity without manual memoization
• Bottom-up state composition with automatic dependency tracking
• Async state with React Suspense integration
• Computed/derived state that auto-updates when dependencies change
• Avoiding prop drilling for shared UI state

Key patterns covered:

• Primitive, derived, write-only, and read-write atoms
• Async atoms with Suspense and loadable/unwrap utilities
• atomWithStorage for persistence
• splitAtom for array item isolation
• Store and Provider patterns for isolation and testing

When NOT to use:

• Server/API data (use your data fetching solution)
• Simple local component state (use useState)
• State that should be in URL (use searchParams)

<philosophy>

Philosophy

Jotai takes an atomic, bottom-up approach to state management. The core principle: "Anything that can be derived from the application state should be derived automatically."

Consider atoms like cells in a spreadsheet:

• Each atom is an independent cell holding a value
• Derived atoms are formulas that reference other cells
• When a cell changes, only formulas depending on it recalculate
• This provides the minimum necessary work automatically

Key characteristics:

• Bottom-up state design: Build state from small, composable atoms
• Automatic dependency tracking: Atoms automatically update when dependencies change
• Fine-grained reactivity: Components only re-render when their specific atoms change
• First-class async support: Async operations integrate seamlessly with React Suspense

Mental Model: Instead of one large store, you have many small atoms that can be combined. This creates natural code splitting and enables precise re-render optimization without manual memoization.

</philosophy>

<patterns>

Core Patterns

Pattern 1: Primitive Atoms

The simplest atom type -- holds a single value with automatic type inference. Always define at module level.

import { atom } from "jotai";

const INITIAL_COUNT = 0;
const countAtom = atom(INITIAL_COUNT);
const userAtom = atom<User | null>(null);

export { countAtom, userAtom };

Why good: Module-level definition persists state, type inference works automatically, explicit typing only for unions/nullable

See [examples/core.md](examples/core.md) Pattern 1 for complete examples with type patterns.

Pattern 2: Derived (Read-Only) Atoms

Compute values from other atoms -- dependencies tracked automatically, cached until dependencies change.

const subtotalAtom = atom((get) => get(priceAtom) * get(quantityAtom));
const taxAtom = atom((get) => get(subtotalAtom) * get(taxRateAtom));
const totalAtom = atom((get) => get(subtotalAtom) + get(taxAtom));

Why good: Automatic dependency tracking, cached computation, chain of derivations is composable

See [examples/core.md](examples/core.md) Pattern 2 for derived atom chains and conditional derivations.

Pattern 3: Write-Only Atoms (Action Atoms)

Encapsulate side effects and multi-atom updates. First argument is null (no read value).

const resetAllAtom = atom(null, (get, set) => {
  set(countAtom, 0);
  set(itemsAtom, []);
  set(selectedAtom, null);
});

Why good: Actions are reusable, enables code splitting, multiple atoms updated atomically

See [examples/core.md](examples/core.md) Pattern 3 for action atoms with arguments.

Pattern 4: Read-Write Atoms

Atoms that can both read derived state and accept writes. Useful for lens-like property access on larger objects.

const nameAtom = atom(
  (get) => get(userAtom).name,
  (get, set, newName: string) => {
    set(userAtom, { ...get(userAtom), name: newName });
  },
);

Why good: Granular read/write access to object properties, keeps parent intact

See [examples/core.md](examples/core.md) Pattern 4 for lens patterns and transformations.

Pattern 5: Async Atoms with Suspense

Async atoms trigger Suspense by default. Use loadable() for manual loading states or unwrap() for fallback values.

// Triggers Suspense -- wrap consumer in <Suspense>
const userAtom = atom(async (get) => {
  const id = get(userIdAtom);
  const response = await fetch(`/api/users/${id}`);
  return response.json() as Promise<User>;
});

// Non-Suspense alternative
const loadableUserAtom = loadable(userAtom);
// Returns { state: 'loading' } | { state: 'hasData', data } | { state: 'hasError', error }

Why good: First-class Suspense integration, loadable provides type-safe discriminated union

See [examples/async.md](examples/async.md) for Suspense setup, loadable/unwrap patterns, and AbortController support.

Pattern 6: atomWithStorage for Persistence

Persist atom values to localStorage, sessionStorage, or custom storage. Use RESET symbol to restore defaults.

import { atomWithStorage, RESET } from "jotai/utils";

const STORAGE_KEY = "app-theme";
const themeAtom = atomWithStorage<Theme>(STORAGE_KEY, "light");
// setTheme(RESET) restores to "light"

Gotcha: Default behavior renders initial value first, then stored value (flicker). Set { getOnInit: true } for immediate stored value, but beware SSR hydration issues.

See [examples/persistence.md](examples/persistence.md) Pattern 1 for storage variants and reset patterns.

Pattern 7: splitAtom for Array Optimization

Split an array atom into individual item atoms. Each item gets its own atom -- updating one item only re-renders that item's component.

import { splitAtom } from "jotai/utils";

const todosAtom = atom<Todo[]>([]);
const todoAtomsAtom = splitAtom(todosAtom, (todo) => todo.id);
// keyExtractor prevents unnecessary atom recreation on reorder

Why good: Per-item re-render isolation, dispatch for add/remove/move operations

See [examples/persistence.md](examples/persistence.md) Pattern 3 for list rendering with dispatch.

Pattern 8: Store and Provider Patterns

Custom stores enable state access outside React, test isolation, and multi-store architectures.

import { createStore, Provider } from "jotai";

const store = createStore();
store.set(countAtom, 10); // Access outside React
store.sub(countAtom, () => { /* react to changes */ });

// Provider with store for isolation
<Provider store={store}><App /></Provider>

Gotcha: Each Provider creates isolated state. Atoms in different Providers do not share values unless given the same store instance.

See [examples/testing.md](examples/testing.md) for store, provider, and test isolation patterns.

</patterns>

Detailed Resources:

• [examples/core.md](examples/core.md) -- Primitive, derived, write-only, and read-write atom patter