How do you decide where each piece of state should live?

Question

IT Interview Practice · Accepted Answer

Deciding **where state lives** is one of the most important architecture skills. The guiding principle: put each piece of state in the **narrowest scope** that still satisfies everyone who needs it — and match each *kind* of state to the right tool.

## A decision framework

```text
1. Who needs this state?
   - One component                  → local state (useState)
   - A few related components        → lift to common parent / small context
   - Many distant components         → global store or context

2. What KIND of state is it?
   - Server/remote data              → server-state library (React Query/SWR)
   - UI/client state                 → useState / Zustand / Redux
   - URL-derivable (filters, page)    → the URL (searchParams)
   - Form input                       → form library or local state

3. How OFTEN does it change?
   - Rarely, widely shared (theme)    → Context is fine
   - Frequently, widely shared        → store with selective subscriptions
```

## The categories and their homes

```text
State type     Lives in                       Examples
─────────────────────────────────────────────────────────────────
Local UI       component (useState)           input text, isOpen, hover
Shared UI      lifted parent / Context / store modal state, selected item
Server data    React Query / SWR / RTK Query  users, products (cached remote)
URL state      router / searchParams          filters, page, search query, tab
Form state     React Hook Form / local         field values, validation
Global app     Zustand / Redux / Context      auth user, theme, cart
```

## Common mistakes the framework prevents

```text
❌ Server data in Redux       → use React Query (it's a cache, not UI state)
❌ Everything global "just in case" → over-coupling, extra re-renders; keep local
❌ Filters/page in state       → put in the URL (shareable, bookmarkable, back-button)
❌ Duplicating derivable values → derive instead (single source of truth)
❌ Form state in a global store → usually overkill; keep local or use a form lib
```

## The URL is underused state

```text
Filters, pagination, search, active tab → belong in the URL.
Benefits: shareable links, browser back/forward works, survives refresh, SEO.
```

## Why it matters

Where state lives determines an app's simplicity, performance, and maintainability.

The skill is resisting the urge to make everything global and instead **localizing by default, matching each kind of state to its proper tool** (server data → React Query, URL-derivable → the URL, UI → local/store).

Getting this right avoids the most common architecture problems — server data crammed into Redux, over-globalized state causing re-renders, and filters in component state instead of the URL.

This judgment is what separates a clean, scalable state architecture from a tangled one.

A decision framework

The categories and their homes

Common mistakes the framework prevents

The URL is underused state

Why it matters

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