tanstack-store— agent skill

Overview

TanStack Store is a lightweight reactive store (signals-like) that powers the internals of TanStack libraries. It provides Store for state, Derived for computed values, Effect for side effects, and batch for atomic updates. Framework adapters provide reactive hooks.

Core: @tanstack/store React: @tanstack/react-store

Installation

npm install @tanstack/store @tanstack/react-store

Store

Creating a Store

import { Store } from '@tanstack/store'

const countStore = new Store(0)

const userStore = new Store<{ name: string; email: string }>({
  name: 'Alice',
  email: '[email protected]',
})

Updating State

// Function updater (immutable update)
countStore.setState((prev) => prev + 1)

userStore.setState((prev) => ({ ...prev, name: 'Bob' }))

Subscribing to Changes

const unsub = countStore.subscribe(() => {
  console.log('Count:', countStore.state)
})

// Cleanup
unsub()

Store Options

const store = new Store(initialState, {
  // Custom update function
  updateFn: (prevValue) => (updater) => {
    return updater(prevValue) // custom logic
  },
  // Callback on subscribe
  onSubscribe: (listener, store) => {
    console.log('New subscriber')
    return () => console.log('Unsubscribed')
  },
  // Callback on every update
  onUpdate: () => {
    console.log('State updated:', store.state)
  },
})

Store Properties

store.state      // Current state
store.prevState  // Previous state
store.listeners  // Set of listener callbacks

Derived (Computed Values)

import { Store, Derived } from '@tanstack/store'

const count = new Store(5)
const multiplier = new Store(2)

const doubled = new Derived({
  deps: [count, multiplier],
  fn: ({ currDepVals }) => currDepVals[0] * currDepVals[1],
})

// MUST mount to activate
const unmount = doubled.mount()

console.log(doubled.state) // 10

count.setState(() => 10)
console.log(doubled.state) // 20

// Cleanup
unmount()

Derived with Previous Value

const accumulated = new Derived({
  deps: [count],
  fn: ({ prevVal, currDepVals }) => {
    return currDepVals[0] + (prevVal ?? 0)
  },
})

Chaining Derived

const filtered = new Derived({
  deps: [dataStore, filterStore],
  fn: ({ currDepVals }) => currDepVals[0].filter(matchesFilter(currDepVals[1])),
})

const sorted = new Derived({
  deps: [filtered, sortStore],
  fn: ({ currDepVals }) => [...currDepVals[0]].sort(comparator(currDepVals[1])),
})

const paginated = new Derived({
  deps: [sorted, pageStore],
  fn: ({ currDepVals }) => currDepVals[0].slice(
    currDepVals[1].offset,
    currDepVals[1].offset + currDepVals[1].limit,
  ),
})

Effect (Side Effects)

import { Store, Effect } from '@tanstack/store'

const count = new Store(0)

const logger = new Effect({
  deps: [count],
  fn: () => {
    console.log('Count changed:', count.state)
    // Optionally return cleanup function
    return () => console.log('Cleaning up')
  },
  eager: false, // true = run immediately on mount
})

const unmount = logger.mount()

count.setState(() => 1) // logs: "Count changed: 1"

unmount()

Effect with Cleanup

const timerEffect = new Effect({
  deps: [intervalStore],
  fn: () => {
    const id = setInterval(() => { /* ... */ }, intervalStore.state)
    return () => clearInterval(id) // cleanup on next run or unmount
  },
})

Batch

Group multiple updates into one notification:

import { batch } from '@tanstack/store'

// Subscribers fire only once with final state
batch(() => {
  countStore.setState(() => 1)
  nameStore.setState(() => 'Alice')
  settingsStore.setState((prev) => ({ ...prev, theme: 'dark' }))
})

React Integration

useStore Hook

import { useStore } from '@tanstack/react-store'

// Subscribe to full state
function Counter() {
  const count = useStore(countStore)
  return <button onClick={() => countStore.setState((c) => c + 1)}>{count}</button>
}

// Subscribe with selector (performance optimization)
function UserName() {
  const name = useStore(userStore, (state) => state.name)
  return <span>{name}</span>
}

// Subscribe to Derived
function DoubledDisplay() {
  const value = useStore(doubledDerived)
  return <span>{value}</span>
}

shallow Equality Function

Prevents re-renders when selector returns structurally-equal objects:

import { useStore } from '@tanstack/react-store'
import { shallow } from '@tanstack/react-store'

function TodoList() {
  // Without shallow: re-renders on ANY state change (new object ref)
  // With shallow: only re-renders when items actually change
  const items = useStore(todosStore, (state) => state.items, shallow)
  return <ul>{items.map(/* ... */)}</ul>
}

Mounting Derived/Effect in React

function MyComponent() {
  useEffect(() => {
    const unmountDerived = myDerived.mount()
    const unmountEffect = myEffect.mount()
    return () => {
      unmountDerived()
      unmountEffect()
    }
  }, [])

  const value = useStore(myDerived)
  return <span>{value}</span>
}

Module-Level Store Pattern

// stores/counter.ts
import { Store, Derived } from '@tanstack/store'

export const counterStore = new Store(0)

export const doubledCount = new Derived({
  deps: [counterStore],
  fn: ({ currDepVals }) => currDepVals[0] * 2,
})

// Actions as plain functions
export function increment() {
  counterStore.setState((c) => c + 1)
}

export function reset() {
  counterStore.setState(() => 0)
}

Framework Adapters

Best Practices

1. Define stores at module level - they're singletons
2. Use selectors in useStore to prevent unnecessary re-renders
3. Use `shallow` when selectors return objects/arrays
4. Always call `mount()` on Derived and Effect instances
5. Always clean up unmount functions (especially in React useEffect)
6. Never mutate state directly - always use setState
7. Use `batch` for multiple related updates
8. Use Derived chains for data transformations (filter -> sort -> paginate)
9. Return cleanup functions from Effect fn for timers/listeners
10. Select primitives when possible (no equality fn needed)

Common Pitfalls

• Forgetting to mount() Derived/Effect (they won't activate)
• Not cleaning up subscriptions/unmount functions (memory leaks)
• Mutating store.state directly instead of using setState
• Creating new object references in selectors without shallow
• Using useStore without a selector (subscribes to everything)
• Forgetting eager: true when Effect should run immediately