React Hooks komplett guide

React Hooks revolutionerade sattet att skriva React-komponenter. Sedan React 16.8 kan du anvanda state, lifecycle och kontext i funktionella komponenter. Denna kompletta React Hooks-guide gar igenom varje Hook med praktiska exempel.

useState: Hantera state

useState ar den mest grundlaggande Hooken.

import { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <button onClick={() => setCount(prev => prev - 1)}>Decrement</button>
    </div>
  );
}

// Lazy initialization — runs only on first render
const [data, setData] = useState(() => {
  return JSON.parse(localStorage.getItem('data') || '{}');
});

// Updating objects — always create a new object
const [user, setUser] = useState({ name: '', age: 0 });
setUser(prev => ({ ...prev, name: 'Alice' }));

// Updating arrays — use spread or filter/map
const [items, setItems] = useState<string[]>([]);
setItems(prev => [...prev, 'new item']);
setItems(prev => prev.filter(item => item !== 'remove me'));

useEffect: Sidoeffekter och lifecycle

useEffect mojliggor sidoeffekter.

import { useEffect, useState } from 'react';

function UserProfile({ userId }: { userId: string }) {
  const [user, setUser] = useState(null);

  // Runs when userId changes (componentDidMount + componentDidUpdate)
  useEffect(() => {
    let cancelled = false;
    fetch(`/api/users/${userId}`)
      .then(res => res.json())
      .then(data => {
        if (!cancelled) setUser(data);
      });

    // Cleanup function (componentWillUnmount)
    return () => { cancelled = true; };
  }, [userId]); // dependency array

  return <div>{user?.name}</div>;
}

// Run once on mount
useEffect(() => {
  console.log('Component mounted');
  return () => console.log('Component unmounted');
}, []); // empty dependency array

// Run on every render (rarely needed)
useEffect(() => {
  console.log('Component rendered');
}); // no dependency array

useContext: Konsumera kontext

useContext prenumererar pa React-kontext.

import { createContext, useContext, useState } from 'react';

// 1. Create a context with a default value
interface ThemeContextType {
  theme: 'light' | 'dark';
  toggleTheme: () => void;
}
const ThemeContext = createContext<ThemeContextType | null>(null);

// 2. Create a provider component
function ThemeProvider({ children }: { children: React.ReactNode }) {
  const [theme, setTheme] = useState<'light' | 'dark'>('light');
  const toggleTheme = () => setTheme(t => t === 'light' ? 'dark' : 'light');

  return (
    <ThemeContext.Provider value={{ theme, toggleTheme }}>
      {children}
    </ThemeContext.Provider>
  );
}

// 3. Consume context with useContext
function ThemeButton() {
  const ctx = useContext(ThemeContext);
  if (!ctx) throw new Error('Must be inside ThemeProvider');

  return (
    <button onClick={ctx.toggleTheme}>
      Current: {ctx.theme}
    </button>
  );
}

// 4. Custom hook for cleaner usage
function useTheme() {
  const ctx = useContext(ThemeContext);
  if (!ctx) throw new Error('useTheme must be used within ThemeProvider');
  return ctx;
}

useMemo: Cacha dyra berakningar

useMemo cachar resultatet av dyra berakningar.

import { useMemo, useState } from 'react';

function ExpensiveList({ items, filter }: { items: Item[]; filter: string }) {
  // Only recalculates when items or filter changes
  const filteredItems = useMemo(() => {
    console.log('Filtering...');
    return items.filter(item =>
      item.name.toLowerCase().includes(filter.toLowerCase())
    );
  }, [items, filter]);

  // Memoize a sorted copy
  const sortedItems = useMemo(() => {
    return [...filteredItems].sort((a, b) => a.name.localeCompare(b.name));
  }, [filteredItems]);

  return (
    <ul>
      {sortedItems.map(item => <li key={item.id}>{item.name}</li>)}
    </ul>
  );
}

// Do NOT overuse useMemo — only for truly expensive computations
// Simple operations do not need memoization
const total = useMemo(() => items.reduce((sum, i) => sum + i.price, 0), [items]);

useCallback: Cacha funktioner

useCallback returnerar en cachad version av en callback.

import { useCallback, useState, memo } from 'react';

// Child component wrapped in memo — only re-renders if props change
const SearchInput = memo(({ onSearch }: { onSearch: (q: string) => void }) => {
  console.log('SearchInput rendered');
  return <input onChange={e => onSearch(e.target.value)} />;
});

function SearchPage() {
  const [query, setQuery] = useState('');
  const [results, setResults] = useState([]);

  // Without useCallback, a new function is created every render
  // causing SearchInput to re-render unnecessarily
  const handleSearch = useCallback((q: string) => {
    setQuery(q);
    fetch(`/api/search?q=${q}`)
      .then(res => res.json())
      .then(setResults);
  }, []); // stable reference

  return (
    <div>
      <SearchInput onSearch={handleSearch} />
      <ul>{results.map(r => <li key={r.id}>{r.title}</li>)}</ul>
    </div>
  );
}

useRef: Muterbara referenser

useRef returnerar ett muterbart ref-objekt.

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

function TextInputWithFocus() {
  // DOM reference
  const inputRef = useRef<HTMLInputElement>(null);

  useEffect(() => {
    inputRef.current?.focus(); // auto-focus on mount
  }, []);

  return <input ref={inputRef} placeholder="Auto-focused" />;
}

function StopWatch() {
  const [time, setTime] = useState(0);
  const intervalRef = useRef<NodeJS.Timeout | null>(null);

  const start = () => {
    intervalRef.current = setInterval(() => {
      setTime(t => t + 1);
    }, 1000);
  };

  const stop = () => {
    if (intervalRef.current) clearInterval(intervalRef.current);
  };

  // Track previous value
  const prevTimeRef = useRef(time);
  useEffect(() => { prevTimeRef.current = time; });

  return (
    <div>
      <p>Time: {time}s (prev: {prevTimeRef.current}s)</p>
      <button onClick={start}>Start</button>
      <button onClick={stop}>Stop</button>
    </div>
  );
}

useReducer: Komplex state-logik

useReducer ar ett alternativ till useState.

import { useReducer } from 'react';

interface State {
  count: number;
  step: number;
}

type Action =
  | { type: 'increment' }
  | { type: 'decrement' }
  | { type: 'reset' }
  | { 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 'reset':
      return { count: 0, step: 1 };
    case 'setStep':
      return { ...state, step: action.payload };
    default:
      return state;
  }
}

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

  return (
    <div>
      <p>Count: {state.count}</p>
      <input
        type="number"
        value={state.step}
        onChange={e => dispatch({ type: 'setStep', payload: Number(e.target.value) })}
      />
      <button onClick={() => dispatch({ type: 'increment' })}>+</button>
      <button onClick={() => dispatch({ type: 'decrement' })}>-</button>
      <button onClick={() => dispatch({ type: 'reset' })}>Reset</button>
    </div>
  );
}

Custom Hooks: Ateranvandbar logik

Custom Hooks extraherar komponentlogik till ateranvandbara funktioner.

// useLocalStorage — persist state to localStorage
function useLocalStorage<T>(key: string, initialValue: T) {
  const [value, setValue] = useState<T>(() => {
    try {
      const item = localStorage.getItem(key);
      return item ? JSON.parse(item) : initialValue;
    } catch {
      return initialValue;
    }
  });

  useEffect(() => {
    localStorage.setItem(key, JSON.stringify(value));
  }, [key, value]);

  return [value, setValue] as const;
}

// useDebounce — debounce a rapidly changing value
function useDebounce<T>(value: T, delay: number): T {
  const [debounced, setDebounced] = useState(value);
  useEffect(() => {
    const timer = setTimeout(() => setDebounced(value), delay);
    return () => clearTimeout(timer);
  }, [value, delay]);
  return debounced;
}

// useFetch — generic data fetching
function useFetch<T>(url: string) {
  const [data, setData] = useState<T | null>(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState<Error | null>(null);

  useEffect(() => {
    let cancelled = false;
    setLoading(true);
    fetch(url)
      .then(res => res.json())
      .then(data => { if (!cancelled) { setData(data); setLoading(false); } })
      .catch(err => { if (!cancelled) { setError(err); setLoading(false); } });
    return () => { cancelled = true; };
  }, [url]);

  return { data, loading, error };
}

// Usage
function App() {
  const [name, setName] = useLocalStorage('name', '');
  const debouncedName = useDebounce(name, 300);
  const { data, loading } = useFetch<User[]>(`/api/search?q=${debouncedName}`);
}

Regler for Hooks

Tva viktiga regler for Hooks.

• Anropa Hooks bara pa toppniva.
• Anropa Hooks bara fran React-funktioner.

// WRONG — Hook inside a condition
function Bad({ isLoggedIn }) {
  if (isLoggedIn) {
    const [user, setUser] = useState(null); // breaks Hook order
  }
}

// CORRECT — condition inside the Hook
function Good({ isLoggedIn }) {
  const [user, setUser] = useState(null);
  useEffect(() => {
    if (isLoggedIn) fetchUser().then(setUser);
  }, [isLoggedIn]);
}

Vanliga fallgropar och losningar

Gamla closures i useEffect

Nar state refereras utan att inkluderas i dependencies.

// BUG: stale closure
function Timer() {
  const [count, setCount] = useState(0);
  useEffect(() => {
    const id = setInterval(() => {
      console.log(count); // always logs 0 (stale!)
      setCount(count + 1); // always sets to 1
    }, 1000);
    return () => clearInterval(id);
  }, []); // count is missing from dependencies
}

// FIX: use functional update
useEffect(() => {
  const id = setInterval(() => {
    setCount(prev => prev + 1); // always uses latest value
  }, 1000);
  return () => clearInterval(id);
}, []); // safe with functional update

Oandliga loopar med useEffect

Satta state i useEffect utan korrekta dependencies.

// BUG: infinite loop
useEffect(() => {
  setCount(count + 1); // triggers re-render, which runs effect again
}); // no dependency array = runs every render

// FIX: add dependency array
useEffect(() => {
  if (count < 10) setCount(count + 1);
}, [count]); // only runs when count changes

Objekt/array-dependencies

Objekt och arrayer jamfors med referens.

// BUG: new object every render
function App() {
  const options = { page: 1, limit: 10 }; // new ref each render
  useEffect(() => {
    fetchData(options);
  }, [options]); // runs every render!
}

// FIX: useMemo to stabilize the reference
function App() {
  const options = useMemo(() => ({ page: 1, limit: 10 }), []);
  useEffect(() => {
    fetchData(options);
  }, [options]); // stable reference
}

Vanliga fragor

React Hooks ar essentiella for modern React-utveckling.