Объясните типизацию React-компонентов и hooks в TypeScript: что это, как работает и когда применять.

// ❌ implicit children, awkward generics
const Avatar: React.FC<Props> = ({ src }) => <img src={src} />;
// ✅ explicit, stricter, generics-friendly
function Avatar({ src }: Props) { return <img src={src} />; }

const [user, setUser] = useState<User | null>(null); // ✅ union is explicit

import { useState, useRef, useReducer } from "react";

// --- Props: type the object, derive children intent explicitly ---
type ButtonProps = {
  variant: "primary" | "ghost";
  onClick: (e: React.MouseEvent<HTMLButtonElement>) => void;
  children: React.ReactNode; // opt IN to children deliberately
};

function Button({ variant, onClick, children }: ButtonProps) {
  return <button className={variant} onClick={onClick}>{children}</button>;
}

// --- Extend native element props without re-typing them all ---
type InputProps = React.ComponentPropsWithoutRef<"input"> & {
  label: string; // your extras on top of every real <input> attribute
};

// --- Refs: element type so .current is typed; null init = read-only DOM ref
const inputRef = useRef<HTMLInputElement>(null);
inputRef.current?.focus(); // ✅ typed, null-guarded

// --- useReducer with a discriminated action union → exhaustive switch ---
type State = { count: number };
type Action = { type: "inc" } | { type: "add"; by: number };

function reducer(state: State, action: Action): State {
  switch (action.type) {
    case "inc": return { count: state.count + 1 };
    case "add": return { count: state.count + action.by }; // action.by is known
  }
}
const [state, dispatch] = useReducer(reducer, { count: 0 });

// --- Generic component: the pattern React.FC makes ugly ---
function List<T>({ items, render }: { items: T[]; render: (item: T) => React.ReactNode }) {
  return <ul>{items.map((it, i) => <li key={i}>{render(it)}</li>)}</ul>;
}