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react-syntax-hooks-basic

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SKILL.md

react-syntax-hooks-basic

Quick Reference

The 7 Essential Hooks

Hook Purpose Returns
useState<S> Local component state [state, setState]
useEffect Side effects after render void
useContext<T> Consume context value T
useRef<T> Mutable ref / DOM access { current: T }
useMemo<T> Memoize expensive values T
useCallback<T> Memoize function references T
useReducer<S, A> Complex state with actions [state, dispatch]

Import

import { useState, useEffect, useContext, useRef, useMemo, useCallback, useReducer } from 'react';

Critical Warnings: Rules of Hooks

NEVER call hooks inside loops, conditions, nested functions, try/catch/finally blocks, or after conditional return statements. React relies on consistent call order to maintain state correctly.

NEVER call hooks from regular JavaScript functions. ALWAYS call hooks from React function components or custom hooks (functions starting with use).

ALWAYS call hooks at the top level of your function component or custom hook — before any early returns.

ALWAYS use eslint-plugin-react-hooks to catch violations automatically.

// WRONG — conditional hook call
function Profile({ userId }: { userId: string | null }) {
  if (!userId) return null;
  const [user, setUser] = useState<User | null>(null); // VIOLATION
}

// CORRECT — hooks before early return
function Profile({ userId }: { userId: string | null }) {
  const [user, setUser] = useState<User | null>(null);
  if (!userId) return null;
  // ... rest of component
}

useState: Local State

Signature

const [state, setState] = useState<S>(initialState: S | (() => S));

Decision Tree

  • Need state that triggers re-render on change? → useState
  • Initial value is expensive to compute? → Pass a function: useState(() => computeExpensive())
  • State depends on previous state? → Use updater function: setState(prev => prev + 1)
  • State is an object or array? → ALWAYS create a new reference when updating

Key Rules

  • setState is stable across renders — safe to omit from dependency arrays.
  • State updates are batched — the screen updates after all event handlers complete.
  • Object.is() comparison — identical values skip re-render.
  • Calling setState does NOT update the variable in the currently executing code.

TypeScript Typing

const [count, setCount] = useState<number>(0);
const [user, setUser] = useState<User | null>(null);
const [items, setItems] = useState<string[]>([]);

React 18 vs 19: No API changes. Behavior consistent across versions.

useEffect: Side Effects

Signature

useEffect(setup: () => (void | (() => void)), dependencies?: unknown[]): void;

Dependency Array Behavior

Pattern Runs when Use case
[dep1, dep2] Any dependency changes Most common — sync to specific values
[] Only on mount (cleanup on unmount) One-time setup (subscriptions, timers)
Omitted Every commit Rarely needed — usually a mistake

Decision Tree

  • Subscribing to external system? → useEffect with cleanup
  • Fetching data? → useEffect with ignore flag for race conditions
  • Need effect before paint? → Use useLayoutEffect instead
  • Transforming data for render? → Do it during render, NOT in an effect

Key Rules

NEVER pass an async function directly as the effect callback. Effects must return void or a cleanup function — async returns a Promise.

// WRONG
useEffect(async () => {
  const data = await fetchData();
}, []);

// CORRECT
useEffect(() => {
  let ignore = false;
  async function fetchData() {
    const result = await api.getData(id);
    if (!ignore) setData(result);
  }
  fetchData();
  return () => { ignore = true; };
}, [id]);

ALWAYS return a cleanup function when your effect creates subscriptions, timers, or event listeners.

useEffect(() => {
  const handler = (e: KeyboardEvent) => { /* ... */ };
  window.addEventListener('keydown', handler);
  return () => window.removeEventListener('keydown', handler);
}, []);
  • Runs AFTER browser paint (non-blocking).
  • Does NOT run during SSR.
  • StrictMode (dev): runs setup → cleanup → setup to catch missing cleanups.

React 18 vs 19: No API changes.

useContext: Context Consumption

Signature

const value = useContext<T>(context: React.Context<T>): T;

Key Rules

  • Returns the value from the closest matching <Provider> above in the tree.
  • Falls back to defaultValue from createContext if no provider found.
  • A provider in the same component does NOT affect useContext in that component.
  • React re-renders ALL consumers when the provider value changes (Object.is comparison).
  • memo() does NOT prevent receiving fresh context values.

TypeScript Typing

interface ThemeContextType {
  theme: 'light' | 'dark';
  toggleTheme: () => void;
}

const ThemeContext = createContext<ThemeContextType | null>(null);

function useTheme(): ThemeContextType {
  const ctx = useContext(ThemeContext);
  if (ctx === null) throw new Error('useTheme must be used within ThemeProvider');
  return ctx;
}

React 18 vs 19: In React 19, use <Context value={...}> directly. In React 18, use <Context.Provider value={...}>.

useRef: Refs and Mutable Values

Signature

const ref = useRef<T>(initialValue: T): { current: T };

Decision Tree

  • Need a DOM element reference? → useRef<HTMLElement>(null)
  • Need a mutable value that does NOT trigger re-render? → useRef
  • Need to store a timeout/interval ID? → useRef
  • Need a value that triggers re-render on change? → Use useState instead

Key Rules

  • Changing .current does NOT trigger a re-render.
  • NEVER read or write .current during render (breaks component purity). Use it in event handlers and effects only.
  • Same object identity across renders.

TypeScript Typing

const inputRef = useRef<HTMLInputElement>(null);
const timerRef = useRef<ReturnType<typeof setTimeout> | null>(null);
const renderCount = useRef<number>(0);

ALWAYS type DOM refs as HTMLElement | null with initial value null:

const divRef = useRef<HTMLDivElement>(null);

React 18 vs 19: No API changes.

useMemo: Memoize Values

Signature

const memoized = useMemo<T>(factory: () => T, deps: unknown[]): T;

Decision Tree

  • Calculation is noticeably slow AND dependencies rarely change? → useMemo
  • Value is passed to a memo()-wrapped child? → useMemo
  • Value is used as a dependency of another hook? → useMemo
  • Calculation is trivial? → Do NOT use useMemo — the overhead outweighs the benefit

Key Rules

  • Factory must be a pure function with no arguments.
  • Dependencies compared via Object.is.
  • React MAY discard the cache — do NOT rely on it as a semantic guarantee.
  • React Compiler (React 19) auto-memoizes, reducing need for manual useMemo.

React 18 vs 19: React Compiler in 19 makes manual useMemo optional in many cases.

useCallback: Memoize Functions

Signature

const memoized = useCallback<T extends (...args: any[]) => any>(fn: T, deps: unknown[]): T;

Decision Tree

  • Passing a function to a memo()-wrapped child? → useCallback
  • Function is a dependency of useEffect or another hook? → useCallback
  • Function is only used in event handlers, not passed down? → Do NOT use useCallback

Equivalence with useMemo

useCallback(fn, deps)  ===  useMemo(() => fn, deps)

Updater Pattern: Remove State Dependencies

// WRONG — deps change every update
const handleAdd = useCallback((text: string) => {
  setTodos([...todos, createTodo(text)]);
}, [todos]);

// CORRECT — empty deps, stable reference
const handleAdd = useCallback((text: string) => {
  setTodos(prev => [...prev, createTodo(text)]);
}, []);

React 18 vs 19: React Compiler in 19 makes manual useCallback optional in many cases.

useReducer: Complex State

Signature

const [state, dispatch] = useReducer<S, A>(
  reducer: (state: S, action: A) => S,
  initialArg: S,
  init?: (arg: S) => S
): [S, Dispatch<A>];

Decision Tree

  • State has multiple sub-values updated together? → useReducer
  • State transitions depend on complex logic? → useReducer
  • Need to pass state update logic down without prop drilling? → useReducer + context
  • Simple boolean or single value? → useState is simpler

Key Rules

  • Reducer MUST be pure — NEVER mutate state, ALWAYS return a new object.
  • dispatch is stable — safe to omit from dependency arrays.
  • Convention: actions are objects with a type property.
  • Lazy init: pass init function as third argument to avoid recreating initial state.

TypeScript Typing

type State = { count: number; step: number };
type Action =
  | { type: 'increment' }
  | { type: 'decrement' }
  | { type: 'setStep'; payload: number };

function reducer(state: State, action: Action): State {
  switch (action.type) {
    case 'increment': return { ...state, count: state.count + state.step };
    case 'decrement': return { ...state, count: state.count - state.step };
    case 'setStep':   return { ...state, step: action.payload };
  }
}

const [state, dispatch] = useReducer(reducer, { count: 0, step: 1 });

React 18 vs 19: No API changes.

Dependency Array Rules

Referential Equality

Dependencies are compared with Object.is. For primitives this is value comparison. For objects, arrays, and functions it is reference comparison.

Dependency type Stable across renders? Solution if unstable
Primitive (string, number, boolean) Yes (same value = same) No action needed
Object/array created in render No (new reference each render) useMemo or move outside
Function created in render No (new reference each render) useCallback or move outside
State setter (setState) Yes (stable identity) Safe to omit
dispatch from useReducer Yes (stable identity) Safe to omit
Ref object from useRef Yes (stable identity) Safe to omit

Common Mistakes

NEVER pass an object or array literal as a dependency — it creates a new reference every render, causing the effect to run on every render:

// WRONG — runs every render
useEffect(() => { /* ... */ }, [{ id: userId }]);

// CORRECT — use the primitive value
useEffect(() => { /* ... */ }, [userId]);

NEVER omit dependencies to "fix" infinite loops — fix the root cause (unstable references) instead.

Reference Links

Official Sources

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