What is the useEffect hook and when is it used?

useEffect is used for side effects in functional components like API calls, subscriptions, or DOM manipulation. It runs after render and can be controlled with dependency arrays.

import { useEffect, useState } from "react";

function UserProfile({ userId }) {
  const [user, setUser] = useState(null);
  
  useEffect(() => {
    fetchUser(userId).then(setUser);
  }, [userId]); // Runs when userId changes
  
  return <div>{user?.name}</div>;
}