Skills
skills/vtexdocs/ai-skills/faststore-state-management

faststore-state-management

Installation
SKILL.md

FastStore SDK State Management

When this skill applies

Use this skill when:

  • You are building any interactive ecommerce feature that involves the shopping cart, user session, product search/filtering, or analytics tracking.
  • You need to add, remove, or update cart items.
  • You need to read or change session data (currency, locale, sales channel, postal code).
  • You need to manage faceted search state (sort order, selected facets, pagination).
  • You are working with @faststore/sdk hooks (useCart, useSession, useSearch).

Do not use this skill for:

  • Visual-only changes — use the faststore-theming skill.
  • Replacing or customizing native components — use the faststore-overrides skill.
  • Extending the GraphQL schema or fetching custom data — use the faststore-data-fetching skill.

Decision rules

  • Use useCart() for all cart operations — it handles platform validation, price verification, and analytics automatically.
  • Use useSession() for all session data — it triggers validateSession mutations that keep the platform synchronized.
  • Use useSearch() within SearchProvider for all search state — it synchronizes with URL parameters and supports browser back-button navigation.
  • Do not build custom state management (React Context, Redux, Zustand, useState/useReducer) for cart, session, or search. The SDK hooks are wired into the FastStore platform integration layer.
  • Always check isValidating from useCart() and block checkout navigation during validation.
  • Use sendAnalyticsEvent() from the SDK for GA4-compatible ecommerce event tracking.

Hard constraints

Constraint: Use @faststore/sdk for Cart, Session, and Search State

MUST use @faststore/sdk hooks (useCart, useSession, useSearch) for managing cart, session, and search state. MUST NOT build custom state management (React Context, Redux, Zustand, useState/useReducer) for these domains.

Why this matters The SDK hooks are wired into the FastStore platform integration layer. useCart() triggers cart validation mutations. useSession() propagates locale/currency changes to all data queries. useSearch() synchronizes with URL parameters and triggers search re-fetches. Custom state bypasses all of this — carts won't be validated, prices may be stale, search won't sync with URLs, and analytics events won't fire.

Detection If you see useState or useReducer managing cart items, cart totals, session locale, session currency, or search facets → STOP. These should use useCart(), useSession(), or useSearch() from @faststore/sdk. If you see createContext with names like CartContext, SessionContext, or SearchContext → STOP. The SDK already provides these contexts.

Correct

// src/components/MiniCart.tsx
import React from 'react'
import { useCart } from '@faststore/sdk'

export default function MiniCart() {
  const { items, totalItems, isValidating, removeItem } = useCart()

  if (totalItems === 0) {
    return <p>Your cart is empty</p>
  }

  return (
    <div data-fs-mini-cart>
      <h3>Cart ({totalItems} items)</h3>
      {isValidating && <span>Updating cart...</span>}
      <ul>
        {items.map((item) => (
          <li key={item.id}>
            <span>{item.itemOffered.name}</span>
            <span>${item.price}</span>
            <button onClick={() => removeItem(item.id)}>Remove</button>
          </li>
        ))}
      </ul>
    </div>
  )
}

Wrong

// WRONG: Building a custom cart context instead of using @faststore/sdk
import React, { createContext, useContext, useReducer } from 'react'

interface CartItem {
  id: string
  name: string
  price: number
  quantity: number
}

// This custom context duplicates what @faststore/sdk already provides.
// Cart changes here will NOT trigger platform validation.
// Prices and availability will NOT be verified against VTEX.
// Analytics events will NOT fire for add-to-cart actions.
const CartContext = createContext<{
  items: CartItem[]
  dispatch: React.Dispatch<any>
}>({ items: [], dispatch: () => {} })

function cartReducer(state: CartItem[], action: any) {
  switch (action.type) {
    case 'ADD':
      return [...state, action.payload]
    case 'REMOVE':
      return state.filter((item) => item.id !== action.payload)
    default:
      return state
  }
}

export function CartProvider({ children }: { children: React.ReactNode }) {
  const [items, dispatch] = useReducer(cartReducer, [])
  return (
    <CartContext.Provider value={{ items, dispatch }}>
      {children}
    </CartContext.Provider>
  )
}

Constraint: Always Handle Cart Validation State

MUST check the isValidating flag from useCart() and show appropriate loading states during cart validation. MUST NOT allow checkout navigation while isValidating is true.

Why this matters Cart validation is an asynchronous operation that checks items against the VTEX platform for current prices, availability, and applicable promotions. If a user proceeds to checkout during validation, they may see stale prices or encounter errors. The isValidating flag exists to prevent this.

Detection If you see useCart() destructured without isValidating in components that have checkout links or "Proceed to Checkout" buttons → warn that the validation state is not being handled. If you see a checkout link or button that does not check isValidating before navigating → warn about potential stale cart data.

Correct

// src/components/CartSummary.tsx
import React from 'react'
import { useCart } from '@faststore/sdk'

export default function CartSummary() {
  const { items, totalItems, isValidating } = useCart()

  const subtotal = items.reduce(
    (sum, item) => sum + item.price * item.quantity,
    0
  )

  return (
    <div data-fs-cart-summary>
      <p>{totalItems} item{totalItems !== 1 ? 's' : ''} in your cart</p>
      <p>Subtotal: ${subtotal.toFixed(2)}</p>
      {isValidating && (
        <p data-fs-cart-validating>Verifying prices and availability...</p>
      )}
      <a
        href="/checkout"
        data-fs-checkout-button
        aria-disabled={isValidating}
        onClick={(e) => {
          if (isValidating) {
            e.preventDefault()
          }
        }}
      >
        {isValidating ? 'Updating cart...' : 'Proceed to Checkout'}
      </a>
    </div>
  )
}

Wrong

// WRONG: Ignoring cart validation state
import React from 'react'
import { useCart } from '@faststore/sdk'

export default function CartSummary() {
  const { items, totalItems } = useCart()
  // Missing isValidating — user can click checkout while cart is being validated.
  // This can lead to price mismatches at checkout or failed orders.

  return (
    <div>
      <p>{totalItems} items</p>
      <a href="/checkout">Proceed to Checkout</a>
      {/* No loading state. No validation check. User may proceed with stale prices. */}
    </div>
  )
}

Constraint: Do Not Store Session Data in localStorage

MUST use useSession() from @faststore/sdk for accessing session data (currency, locale, channel, person). MUST NOT read/write session data directly to localStorage or sessionStorage.

Why this matters The SDK's session module manages synchronization with the VTEX platform. When session data changes, the SDK triggers a validateSession mutation that updates the server-side session and re-validates the cart. Writing directly to localStorage bypasses this validation — the platform won't know about the change, prices may display in the wrong currency, and cart items may not reflect the correct sales channel.

Detection If you see localStorage.getItem or localStorage.setItem with keys related to session data (currency, locale, channel, region, postalCode) → STOP. These should be managed through useSession(). If you see sessionStorage used for the same purpose → STOP.

Correct

// src/components/LocaleSwitcher.tsx
import React from 'react'
import { useSession } from '@faststore/sdk'

export default function LocaleSwitcher() {
  const { locale, currency, setSession } = useSession()

  const handleLocaleChange = (newLocale: string, newCurrency: string) => {
    // setSession triggers platform validation and re-fetches data
    setSession({
      locale: newLocale,
      currency: { code: newCurrency, symbol: newCurrency === 'USD' ? '$' : 'R$' },
    })
  }

  return (
    <div data-fs-locale-switcher>
      <button
        onClick={() => handleLocaleChange('en-US', 'USD')}
        aria-pressed={locale === 'en-US'}
      >
        EN
      </button>
      <button
        onClick={() => handleLocaleChange('pt-BR', 'BRL')}
        aria-pressed={locale === 'pt-BR'}
      >
        PT
      </button>
      <span>Current: {locale} ({currency.code})</span>
    </div>
  )
}

Wrong

// WRONG: Managing session data manually via localStorage
import React, { useState, useEffect } from 'react'

export default function LocaleSwitcher() {
  const [locale, setLocale] = useState('en-US')

  useEffect(() => {
    // WRONG: Reading session data from localStorage
    const saved = localStorage.getItem('store-locale')
    if (saved) setLocale(saved)
  }, [])

  const handleLocaleChange = (newLocale: string) => {
    // WRONG: Writing session data to localStorage
    // The VTEX platform does NOT know about this change.
    // Product prices, availability, and cart will NOT update.
    localStorage.setItem('store-locale', newLocale)
    setLocale(newLocale)
  }

  return (
    <div>
      <button onClick={() => handleLocaleChange('en-US')}>EN</button>
      <button onClick={() => handleLocaleChange('pt-BR')}>PT</button>
    </div>
  )
}

Preferred pattern

Recommended usage of SDK hooks together:

// src/components/CartDrawer.tsx
import React from 'react'
import { useCart } from '@faststore/sdk'
import { useSession } from '@faststore/sdk'
import { Button, Loader } from '@faststore/ui'

export default function CartDrawer() {
  const { items, totalItems, isValidating, removeItem, updateItemQuantity } =
    useCart()
  const { currency, locale } = useSession()

  const formatter = new Intl.NumberFormat(locale, {
    style: 'currency',
    currency: currency.code,
  })

  const subtotal = items.reduce(
    (sum, item) => sum + item.price * item.quantity,
    0
  )

  if (totalItems === 0) {
    return (
      <div data-fs-cart-drawer>
        <h2>Your Cart</h2>
        <p>Your cart is empty. Start shopping to add items.</p>
      </div>
    )
  }

  return (
    <div data-fs-cart-drawer>
      <h2>Your Cart ({totalItems} items)</h2>

      {isValidating && (
        <div data-fs-cart-loading>
          <Loader />
          <span>Verifying prices and availability...</span>
        </div>
      )}

      <ul data-fs-cart-items>
        {items.map((item) => (
          <li key={item.id} data-fs-cart-item>
            <span data-fs-cart-item-name>{item.itemOffered.name}</span>
            <span data-fs-cart-item-price>{formatter.format(item.price)}</span>
            <div data-fs-cart-item-quantity>
              <Button
                variant="tertiary"
                onClick={() => updateItemQuantity(item.id, item.quantity - 1)}
                disabled={item.quantity <= 1}
              >
                -
              </Button>
              <span>{item.quantity}</span>
              <Button
                variant="tertiary"
                onClick={() => updateItemQuantity(item.id, item.quantity + 1)}
              >
                +
              </Button>
            </div>
            <Button variant="tertiary" onClick={() => removeItem(item.id)}>
              Remove
            </Button>
          </li>
        ))}
      </ul>

      <div data-fs-cart-summary>
        <span>Subtotal: {formatter.format(subtotal)}</span>
        <a
          href="/checkout"
          data-fs-checkout-button
          aria-disabled={isValidating}
          onClick={(e) => {
            if (isValidating) e.preventDefault()
          }}
        >
          {isValidating ? 'Updating cart...' : 'Proceed to Checkout'}
        </a>
      </div>
    </div>
  )
}

Search state with useSearch():

// src/components/FacetFilter.tsx
import { useSearch } from '@faststore/sdk'

export default function FacetFilter() {
  const { state, setState } = useSearch()

  const toggleFacet = (facetKey: string, facetValue: string) => {
    const currentFacets = state.selectedFacets || []
    const exists = currentFacets.some(
      (f) => f.key === facetKey && f.value === facetValue
    )

    const newFacets = exists
      ? currentFacets.filter(
          (f) => !(f.key === facetKey && f.value === facetValue)
        )
      : [...currentFacets, { key: facetKey, value: facetValue }]

    setState({
      ...state,
      selectedFacets: newFacets,
      page: 0, // Reset pagination when filters change
    })
  }

  return (
    <div data-fs-facet-filter>
      <button onClick={() => toggleFacet('brand', 'Nike')}>
        Nike {state.selectedFacets?.some((f) => f.key === 'brand' && f.value === 'Nike') ? '✓' : ''}
      </button>
    </div>
  )
}

Common failure modes

  • Creating a custom React Context for cart state (CartContext, useReducer) — disconnects from VTEX platform validation, analytics, and checkout.
  • Storing session data (locale, currency, postal code) in localStorage — the SDK's validateSession mutation never fires, so the platform is out of sync.
  • Building custom search state with useState — loses URL synchronization, breaks back-button navigation, and bypasses the SDK's optimized query generation.
  • Ignoring the isValidating flag from useCart() — users can proceed to checkout with stale prices or out-of-stock items.
  • Using useCart_unstable or useSession_unstable hooks without understanding they have unstable interfaces that may change.

Review checklist

  • Is cart state managed exclusively via useCart() from @faststore/sdk?
  • Is session data accessed exclusively via useSession() from @faststore/sdk?
  • Is search state managed via useSearch() within a SearchProvider context?
  • Is the isValidating flag checked before allowing checkout navigation?
  • Is there no custom React Context, Redux, or Zustand store duplicating SDK state?
  • Is there no direct localStorage/sessionStorage access for session-related data?

Reference

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