useEffect

import { useState, useEffect } from 'react';

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

    // Run on mount and when userId changes
    useEffect(() => {
        let cancelled = false;

        fetch(`/api/users/${userId}`)
            .then(res => res.json())
            .then(data => {
                if (!cancelled) setUser(data);
            });

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

    if (!user) return <div>Loading...</div>;
    return <div>{user.name}</div>;
}

// Event listener
function WindowSize() {
    const [size, setSize] = useState(window.innerWidth);

    useEffect(() => {
        const handler = () => setSize(window.innerWidth);
        window.addEventListener('resize', handler);
        return () => window.removeEventListener('resize', handler);
    }, []);

    return <div>Width: {size}px</div>;
}

// Timer
function Timer() {
    const [seconds, setSeconds] = useState(0);

    useEffect(() => {
        const interval = setInterval(() => {
            setSeconds(s => s + 1);
        }, 1000);
        return () => clearInterval(interval);
    }, []);

    return <div>{seconds}s</div>;
}

// Run only on mount
function MountOnly() {
    useEffect(() => {
        console.log('Mounted');
    }, []);
    return <div>Hello</div>;
}