React Hooks Mastery

Patterns for writing correct, performant, and composable React hooks.

When to Use

• User is creating custom hooks
• User asks about useCallback, useMemo, or performance optimization
• User has complex state logic that needs useReducer
• User is integrating an external store (Redux, Zustand, vanilla)
• User encounters stale closure bugs or rules of hooks violations

Core Patterns

Custom Hooks -- Extracting Reusable Logic

Custom hooks encapsulate stateful logic for reuse across components. Name them with the use prefix.

import { useState, useEffect, useRef } from 'react'

function useFetch<T>(url: string) {
  const [data, setData] = useState<T | null>(null)
  const [error, setError] = useState<Error | null>(null)
  const [isLoading, setIsLoading] = useState(true)

  useEffect(() => {
    const controller = new AbortController()
    setIsLoading(true)
    setError(null)

    fetch(url, { signal: controller.signal })
      .then((res) => {
        if (!res.ok) throw new Error(`HTTP ${res.status}`)
        return res.json()
      })
      .then((json) => setData(json as T))
      .catch((err) => {
        if (err.name !== 'AbortError') setError(err)
      })
      .finally(() => setIsLoading(false))

    return () => controller.abort()
  }, [url])

  return { data, error, isLoading }
}

useCallback and useMemo -- When to Memoize

Memoize only when passing callbacks to memoized children or when computation is expensive. Do not memoize everything by default.

import { useCallback, useMemo } from 'react'

function ProductList({ products, onSelect }: Props) {
  // Memoize because this is passed to React.memo children
  const handleSelect = useCallback(
    (id: string) => {
      onSelect(id)
    },
    [onSelect]
  )

  // Memoize because sorting is O(n log n)
  const sorted = useMemo(
    () => [...products].sort((a, b) => a.price - b.price),
    [products]
  )

  return (
    <ul>
      {sorted.map((p) => (
        <ProductItem key={p.id} product={p} onSelect={handleSelect} />
      ))}
    </ul>
  )
}

const ProductItem = React.memo(({ product, onSelect }: ItemProps) => (
  <li onClick={() => onSelect(product.id)}>{product.name}</li>
))

useRef -- Mutable Values Without Re-renders

Use refs for values that should persist across renders without triggering re-renders: timers, previous values, DOM elements.

function useInterval(callback: () => void, delay: number | null) {
  const savedCallback = useRef(callback)

  // Update ref on every render so the interval always calls the latest callback
  useEffect(() => {
    savedCallback.current = callback
  }, [callback])

  useEffect(() => {
    if (delay === null) return
    const id = setInterval(() => savedCallback.current(), delay)
    return () => clearInterval(id)
  }, [delay])
}

function usePrevious<T>(value: T): T | undefined {
  const ref = useRef<T | undefined>(undefined)
  useEffect(() => {
    ref.current = value
  })
  return ref.current
}

useReducer -- Complex State Transitions

Prefer useReducer when state transitions depend on previous state or when multiple sub-values change together.

import { useReducer } from 'react'

interface FormState {
  values: Record<string, string>
  errors: Record<string, string>
  isSubmitting: boolean
}

type FormAction =
  | { type: 'SET_FIELD'; field: string; value: string }
  | { type: 'SET_ERROR'; field: string; error: string }
  | { type: 'SUBMIT_START' }
  | { type: 'SUBMIT_SUCCESS' }
  | { type: 'SUBMIT_FAILURE'; errors: Record<string, string> }

function formReducer(state: FormState, action: FormAction): FormState {
  switch (action.type) {
    case 'SET_FIELD':
      return {
        ...state,
        values: { ...state.values, [action.field]: action.value },
        errors: { ...state.errors, [action.field]: '' },
      }
    case 'SET_ERROR':
      return { ...state, errors: { ...state.errors, [action.field]: action.error } }
    case 'SUBMIT_START':
      return { ...state, isSubmitting: true }
    case 'SUBMIT_SUCCESS':
      return { ...state, isSubmitting: false, errors: {} }
    case 'SUBMIT_FAILURE':
      return { ...state, isSubmitting: false, errors: action.errors }
  }
}

function useForm(initialValues: Record<string, string>) {
  const [state, dispatch] = useReducer(formReducer, {
    values: initialValues,
    errors: {},
    isSubmitting: false,
  })

  const setField = (field: string, value: string) =>
    dispatch({ type: 'SET_FIELD', field, value })

  const submit = async (handler: (values: Record<string, string>) => Promise<void>) => {
    dispatch({ type: 'SUBMIT_START' })
    try {
      await handler(state.values)
      dispatch({ type: 'SUBMIT_SUCCESS' })
    } catch (err) {
      dispatch({ type: 'SUBMIT_FAILURE', errors: { form: String(err) } })
    }
  }

  return { ...state, setField, submit }
}

useSyncExternalStore -- Subscribing to External State

The correct way to subscribe React to any external mutable store (vanilla stores, browser APIs, third-party state).

import { useSyncExternalStore } from 'react'

function createStore<T>(initialState: T) {
  let state = initialState
  const listeners = new Set<() => void>()

  return {
    getState: () => state,
    setState: (next: T | ((prev: T) => T)) => {
      state = typeof next === 'function' ? (next as (prev: T) => T)(state) : next
      listeners.forEach((listener) => listener())
    },
    subscribe: (listener: () => void) => {
      listeners.add(listener)
      return () => listeners.delete(listener)
    },
  }
}

const counterStore = createStore({ count: 0 })

function useCounter() {
  const state = useSyncExternalStore(
    counterStore.subscribe,
    counterStore.getState,
    counterStore.getState // server snapshot
  )
  return {
    count: state.count,
    increment: () => counterStore.setState((s) => ({ count: s.count + 1 })),
  }
}

Anti-Patterns

• Calling hooks conditionally -- Hooks must be called in the same order every render. Never put hooks inside if/else, loops, or after early returns.
• Missing dependencies in useEffect -- Causes stale closures and subtle bugs. Always include all referenced values in the dependency array. Use the exhaustive-deps ESLint rule.
• Memoizing everything -- useCallback and useMemo have overhead. Only memoize when passing to React.memo children or for genuinely expensive computations.
• Using useRef to avoid dependency arrays -- This hides bugs. Fix the dependency array instead of working around it with refs (except for the interval/timer pattern).
• Giant useEffect -- An effect that does multiple unrelated things. Split into separate useEffect calls, each with its own dependencies.

Quick Reference

Rules of Hooks: (1) Only call at the top level. (2) Only call from React functions or custom hooks.