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React component discipline: pure components, minimal state, effects as escape hatches. Invoke whenever task involves any interaction with React code — writing, reviewing, refactoring, debugging, or understanding JSX, hooks, component architecture, state management, or performance optimization.
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
Components — ${CLAUDE_SKILL_DIR}/references/components.md Composition, refs, metadata, custom elements
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.
Component Structure
One component per file. Small helpers co-located in the same file are acceptable but extract when reused.
Prefer function declarations for components.
Do not use React.FC — it adds implicit children typing and complicates generics. Use
function Component(props: Props).
Component Body Organization
Separate logic from rendering. The component body handles computation, state, and handler definitions. JSX is
declarative — it references results, not processes.
Handler object — group all event handlers in a single handle object. This creates a clear boundary between logic
and rendering:
Reference in JSX as onChange={handle.inputChange}. Never inline handler logic in JSX.
Pre-render computation — move list rendering and derived JSX out of the return statement into component body
variables:
consttabElements: ReactNode[] = [];
for (const tab of allTabs) {
tabElements.push(<Tabkey={tab.id}>{tab.name}</Tab>);
}
return<TabList>{tabElements}</TabList>;
JSX .map() inside the return statement is discouraged — compute element arrays in the body, reference them in JSX.
Conditional rendering — simple conditions ({isVisible && <Component />}) are acceptable inline in JSX. When the
condition is complex or involves multiple branches, compute the result in the component body and reference the
variable in JSX.
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.
Never inline handler logic in JSX. Group all handlers in a handle object in the component body (see Component Body
Organization).
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:
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)
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
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.
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
Every component with props must have a dedicated named type (e.g., ButtonProps). Never define prop types inline in
the function signature. Use 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 ${CLAUDE_SKILL_DIR}/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 ${CLAUDE_SKILL_DIR}/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.
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.
