React

Components are pure functions. State is minimal. Effects are escape hatches. If you reach for useEffect, verify you actually need it.

React rewards thinking in components: break UI into pieces, find minimal state, identify where it lives, and wire data flow from parent to child. References contain extended examples, rationale, and edge cases for each topic area.

References

Topic	Reference	Contents
Components	`references/components.md`	Composition, refs, metadata, custom elements
Hooks	`references/hooks.md`	Hook rules, custom hooks, `useSyncExternalStore`
State	`references/state.md`	Placement, reducers, context, actions
Performance	`references/performance.md`	Compiler, memoization, server components, streaming
Testing	`references/testing.md`	Query priority/variants, userEvent catalog, async patterns

Component Design

Thinking in React

Build UI in five steps:

Break UI into a component hierarchy — each component does one thing.
Build a static version first — props only, no state, no interactivity.
Find minimal state — if it doesn't change, is passed from a parent, or can be computed, it is not state.
Identify where state lives — find every component that renders based on the state, find their closest common parent, put state there.
Add inverse data flow — pass state-setter callbacks down so children update parent state through event handlers.

Purity

React assumes every component is a pure function. Same props + same state = same JSX. Never mutate props, state, or variables declared before rendering.

Local mutation is fine. Creating and mutating objects/arrays within the same render is safe — the mutation is invisible outside that render.
Event handlers don't need to be pure — they run outside of rendering.

Side effect type	Where it belongs
User clicks, form submits	Event handlers
Sync with external system (DOM, network)	`useEffect` (last resort)
Data transformation	Compute during render
Shared logic between handlers	Extract a function, call from handlers

Component Structure

One component per file. Small helpers co-located in the same file are acceptable but extract when reused.
Always prefer function declarations for components.
Do not use React.FC — it adds implicit children typing and complicates generics. Use function Component(props: Props).

Composition

Props flow down, events flow up. One-way data flow. Children never mutate parent state directly — they call callbacks.
Composition over configuration. Pass JSX as children or render props instead of building components with dozens of boolean flags.
When a wrapper component updates its own state, React knows its children props haven't changed, so children skip re-rendering.
Use compound components (shared context between related sub-components) for complex UI patterns like flyout menus, tabs, accordions.
Prefer controlled components when parent needs to coordinate state across siblings. Prefer uncontrolled for isolated, self-contained UI.

Refs

ref is a prop. Pass ref directly as a prop to function components. Never use forwardRef — it is deprecated.
Ref callbacks can return a cleanup function, called when the element unmounts.
Avoid implicit returns in ref callbacks — use block body {} not parentheses to prevent TypeScript confusion.

Document Metadata

Render <title>, <meta>, <link> directly in components. React hoists them to <head> automatically. Works with client-only apps, streaming SSR, and Server Components.

Custom Elements

React provides full custom element support. Server rendering: primitive props render as attributes, non-primitive props are omitted. Client rendering: props matching element instance properties are assigned as properties, others as attributes.

JSX Conventions

Self-closing tags for components without children: <Input />.
Boolean attributes without value: <Input disabled /> not disabled={true}.
Fragments to avoid wrapper divs: <>...</> or <Fragment key={id}>.
Avoid && with numbers — count && <List /> renders 0. Use count > 0 && <List /> or a ternary.
Inline event handlers are fine for simple one-liners. Extract to named functions when logic is complex.
Spread props sparingly — {...props} makes it unclear what a component accepts. Prefer explicit props.
key on every list item. Stable, unique identifiers. Never use array index as key when items can reorder.
No side effects during render. Event handlers for user actions, Effects for synchronization, render for pure computation.

Hooks

`use()` — Context and Promises

Prefer use(MyContext) over useContext(MyContext). use() can be called inside conditionals and loops — useContext() cannot.
use() always looks for the closest provider above the calling component.
use(promise) integrates with Suspense and Error Boundaries to read promise values.
Do not create promises inside Client Components during render — they recreate every render. Pass promises from Server Components or use a Suspense-compatible library.
In Server Components, prefer async/await over use().
use() cannot be called in a try-catch block. Use Error Boundaries or promise.catch() instead.

Hook Rules

Top level only. Never call hooks inside conditions, loops, or nested functions. React relies on call order. Exception: use() can be called conditionally.
React functions only. Call hooks from function components or custom hooks — never from regular JavaScript functions.
Exhaustive deps. Include all reactive values used inside the Effect in the dependency array. The linter enforces this — don't suppress it. If a dependency causes unwanted re-runs, restructure the code.
One Effect per concern. Don't merge unrelated sync logic into a single Effect. Separate Effects for separate external systems.

Effects Are Escape Hatches

Use Effects only to synchronize with external systems (DOM APIs, network, browser events). Not for transforming data, handling user events, or state derivation.

You don't need an Effect for:

Situation	Do this instead
Transform data for rendering	Compute during render
Handle user events	Call in event handler
Reset state on prop change	Use `key={userId}` on the component
Adjust state on prop change	Compute: `items.find(...)` during render
Notify parent of state change	Call `onChange` in the event handler
Share logic between handlers	Extract a function, call from both handlers
Chain state updates	Calculate all state in one event handler

You DO need an Effect for:

Subscribing to browser events (online/offline, resize, intersection)
Connecting to external systems (WebSocket, third-party widgets)
Fetching data that depends on current props/state (with cleanup)
Synchronizing with non-React DOM (imperative animations, canvas)

Every Effect that subscribes must return a cleanup function. Data fetching in Effects must use a cleanup flag (let ignore = false) to prevent race conditions. Prefer a data-fetching library or use() with Suspense over raw Effects for fetching.

Custom Hooks

Custom hooks share stateful logic, not state itself. Each call creates independent state.
Must start with use followed by a capital letter. Functions that don't call hooks should NOT start with use.
Name after the use case, not the lifecycle — useOnlineStatus not useMount.
Extract when: repetitive Effect logic across components, complex state + Effect combos, or synchronization with external systems.
Don't extract a single useState into a hook — that's unnecessary abstraction.

Return value conventions:

Single value: return directly (return isOnline)
Value + setter pair: return tuple (return [value, setValue] as const)
Multiple related values: return object (return { value, onChange, reset })

`useSyncExternalStore`

For subscribing to external data stores, prefer useSyncExternalStore over manual Effect + state. Provide a subscribe function, a client snapshot getter, and a server snapshot getter for SSR.

State Management

State Placement Decision Tree

Does only one component use it? Keep it local with useState.
Do siblings need it? Lift to their closest common parent.
Is prop drilling painful (5+ levels)? Try composition first — restructure components to pass JSX as children.
Still painful after composition? Use Context with use().
Is it server data? Use a data-fetching library — not useState + useEffect.

Local state → Lift state up → Composition → Context → External library

Server Cache vs UI State

Category	Examples	Tool
UI state	Modal open, form input, selected tab	`useState`, `useReducer`, Context
Server cache	User data, search results, API responses	react-query, SWR, framework loaders

Never reinvent caching, deduplication, and race condition handling with raw useState.

`useState`

Minimal state. If it can be computed from existing props or state, compute it during render — don't store it.
Colocate state. Keep state as close to where it's used as possible. Lift up only when multiple components need it.
Use updater functions when next state depends on previous state: setCount(prev => prev + 1) not setCount(count + 1).
Lazy initialization for expensive initial values — pass a function: useState(() => createInitialState()) not useState(createInitialState()).

`useReducer`

Use when state updates are complex — many event handlers modifying the same state, or when next state depends on previous state in non-trivial ways.

Signal	Tool
Single value, simple updates	`useState`
Multiple related values, complex transitions	`useReducer`
Many event handlers doing similar state updates	`useReducer`
Need to test state logic in isolation	`useReducer`

Reducer rules:

Reducers must be pure — same inputs = same output, no side effects.
Each action describes a single user interaction — dispatch reset_form not five separate set_field actions.
Actions describe what happened, not what to do — 'added_task' not 'set_tasks'.
Always have a default case that throws to catch typos early.
Use as const for action types in TypeScript.

`key` for Identity Reset

Use key to reset a component's state when the conceptual entity changes: <Profile key={userId} />. This is cleaner than using an Effect to reset state on prop change.

Context

Use <Context value={...}> directly — not <Context.Provider value={...}>.
Always wrap context consumption in a custom hook with a null check that throws if used outside the provider.
Try props and composition first. Context is not the first solution to prop drilling.
Keep context close to where it's used — not every context belongs at the app root.
Split logically — user settings separate from notifications. Don't put all state in one giant context.
Different createContext() calls are independent — they don't override each other.
For complex shared state, combine useReducer with context. Split into two contexts (data + dispatch) so components that only dispatch don't re-render on data changes.

Actions

Use useActionState for form submissions and data mutations. It manages pending state, errors, and sequential action queuing automatically.
When passed to <form action>, React wraps submission in a transition automatically. When calling dispatch manually, wrap in startTransition.
Return error states instead of throwing to prevent skipping queued actions.
Use useOptimistic for instant UI feedback while async Actions complete. The optimistic state reverts to real value when the Action completes or fails.
Use a reducer form of useOptimistic for complex updates (e.g., adding to a list).
useFormStatus reads submission status of the nearest parent <form> — must be called from a component rendered inside a <form>, not in the same component.

TypeScript

Type props with interfaces. function Button(props: ButtonProps) or destructure: function Button({ label }: ButtonProps).
Don't use React.FC. Use plain function declarations.
Type events explicitly when needed: (e: React.ChangeEvent<HTMLInputElement>) => void.
Use as const for action types in reducers.

Performance

React Compiler

React Compiler is a build-time tool that automatically applies memo, useMemo, and useCallback equivalents. When using the compiler:

Do not manually wrap components in memo, use useMemo, or useCallback in new code.
Leave existing memoization in place — removing it can change compilation output.
Use manual memoization only as an escape hatch (e.g., stabilizing an Effect dependency).
The compiler works with plain JavaScript and the Rules of React — no code changes needed.

Optimization Decision Tree

Is there a perceptible lag? No — don't optimize.
Is the render itself slow? Profile it. Fix the computation.
Are components re-rendering unnecessarily? Restructure first (push state down, lift content up).
Still slow after restructuring? Apply memo, useMemo, useCallback.

Fix the slow render before you fix the re-render. Restructuring beats memoization.

Manual Memoization (When Compiler Is Unavailable)

memo(Component) — skip re-rendering when props unchanged. Useful when: component re-renders often with same props, re-rendering is expensive, parent re-renders for unrelated reasons. Useless when: props always differ, component is cheap, or it re-renders from its own state/context anyway.
useMemo(fn, deps) — cache computed values. Only for genuinely expensive work or preserving references passed to memoized children.
useCallback(fn, deps) — cache function references. Use when passing callbacks to memoized children, in custom hooks returning functions, or as Effect dependencies.

Server Components

Server Components render on the server, send only output to client. No client JS.
Can read databases, filesystems, APIs directly. Can be async functions.
Cannot use useState, useEffect, or any client-side React APIs.
Default (no directive needed). Client Components require "use client" at file top.
Server Components can render Client Components as children. Client Components cannot import Server Components directly.
"use server" marks Server Functions (Actions callable from client), not Server Components.
Automatic code-splitting: Client Component imports from Server Components are code-split automatically.

Streaming with Suspense

Start rendering immediately, stream slower parts as they resolve. Create promises in Server Components, pass to Client Components, read with use() inside <Suspense>.

Bundle Optimization

Avoid barrel file imports — import { Button } from '@/components/Button' not from '@/components'.
Use lazy(() => import('./Chart')) with <Suspense> for heavy components.
Parallelize independent data fetches with Promise.all, never sequential awaits.

Error Handling

Use onCaughtError and onUncaughtError root options on createRoot for fine-grained error reporting — caught errors come from Error Boundaries, uncaught from unhandled throws.

Testing

Philosophy

Tests resemble how users interact with the application. Query by what users see (roles, text, labels), not by implementation details (class names, component internals, test IDs).

Setup

Always use screen for queries — never destructure from render(). Set up userEvent.setup() before rendering.

Queries

Use the highest-priority query that works: getByRole > getByLabelText > getByText > getByTestId (last resort). Use getBy for present elements, queryBy for asserting absence, findBy for async appearance. Full query priority and variant tables in references/testing.md.

User Interactions

Always prefer userEvent over fireEvent — it simulates real user behavior (focus, blur, keyDown/keyPress/keyUp sequence). Full method catalog in references/testing.md.

Async Patterns

Use waitFor for assertions that need to wait for async operations.
One assertion per waitFor callback — multiple assertions cause slower failure detection.
Never put side-effects in waitFor — the callback may run multiple times.
Never pass an empty callback to waitFor.
Prefer findBy over waitFor + getBy.

Testing Actions and Forms

Render the component and interact as a user would. For useFormStatus components, ensure the component is rendered inside a <form> with an action prop.

Rules

Don't wrap in act unnecessarily — render() and fireEvent already handle it. If you see act warnings, fix the root cause (state update after test finishes).
Don't add role attributes to native elements — <button> already has role="button".
Make inputs accessible with type and <label> — this makes them queryable by role.
If you can't query by role, the element is probably not accessible to screen readers.
Don't call cleanup manually — it's automatic.
Use @testing-library/jest-dom matchers: toBeInTheDocument(), toBeVisible(), toBeDisabled(), toHaveTextContent(), toHaveAttribute(), toHaveValue().

Application

When writing React code:

Apply all conventions silently — don't narrate each rule.
If an existing codebase contradicts a convention, follow the codebase and flag the divergence once.
Always prefer function declarations for components.

When reviewing React code:

Cite the specific violation and show the fix inline.
Don't lecture or quote the rule — state what's wrong and how to fix it.

Integration

This skill provides React-specific conventions. The coding skill governs workflow; language skills govern JS/TS choices; this skill governs component architecture, hooks, state management, and rendering discipline.

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