$ npm install react@18.1.0 react-dom@18.1.0
or
$ yarn add react@18.1.0 react-dom@18.1.0

// <= v17
//// client render
import { render } from 'react-dom';
const container = document.getElementById('app');
render(<App tab="home" />, container);

//// unmount
unmountComponentAtNode(container);

//// render callback(렌더링 완료 이후에 호출되는 콜백함수)
const container = document.getElementById('app');
render(<App tab="home" />, container, () => {
  console.log('rendered');
});

//// hydration
import { hydrate } from 'react-dom';
const container = document.getElementById('app');
hydrate(<App tab="home" />, container);

// >= 18
//// client render
import { createRoot } from 'react-dom/client';
const container = document.getElementById('app');
const root = createRoot(container); // createRoot(container!) if you use TypeScript
root.render(<App tab="home" />);

//// unmount
root.unmount();

//// render callback(useEffect)
function AppWithCallbackAfterRender() {
  useEffect(() => {
    console.log('rendered');
  });

  return <App tab="home" />;
}

//// hydration
import { hydrateRoot } from 'react-dom/server';
const container = document.getElementById('app');
root.hydrateRoot(container, <App tab="home" />);

// v17 이전
ReactDOM.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>,
  document.getElementById('root'),
);

// v18 이후(createRoot)
ReactDOM.createRoot(document.getElementById('root') as HTMLDivElement).render(
  <React.StrictMode>
    <App />
  </React.StrictMode>,
);

// <= 17
// 리액트 이벤트 Callback 함수에서는 배칭 방식으로 상태를 모아서 업데이트 진행
function App() {
  const [count, setCount] = useState(0);
  const [flag, setFlag] = useState(false);

  function handleClick() {
    setCount((c) => c + 1);
    setFlag((f) => !f);
  }

  console.log("render");
  return (
    <div>
      <button onClick={handleClick}>Next</button>
      <h1>{count}</h1>
    </div>
  );}

const rootElement = document.getElementById("root");
// This opts into the new behavior!
// ReactDOM.createRoot(rootElement).render(<App />); -> 콘솔에 렌더가 1번 찍힘(>= 18)
// This keeps the old behavior:
ReactDOM.render(<App />, rootElement); -> 콘솔에 렌더가 2번 찍힘

// >= 18
// setTimeOut
setTimeout(() => {
  setCount(c => c + 1);
  setFlag(f => !f);
  // React는 이 함수가 끝날 때만 리렌더링을 한다 (배칭이다!)
}, 1000);

// Promise
fetch(/*...*/).then(() => {
  setCount(c => c + 1);
  setFlag(f => !f);
  // React는 이 함수가 끝날 때만 리렌더링을 한다 (배칭이다!)
});

// native event handler
elm.addEventListener('click', () => {
  setCount(c => c + 1);
  setFlag(f => !f);
  // React는 이 함수가 끝날 때만 리렌더링을 한다 (배칭이다!)
});

import { flushSync } from 'react-dom'; // Note: react가 아닌 react-dom이다

// 뱃칭을 원치 않는 경우(state 변경 후 dom에서 데이터를 가져와야하는 경우)
function handleClick() {
  flushSync(() => {
    setCounter(c => c + 1);
  });
  // 이 과정이 끝났을 때 React는 DOM을 업데이트한 상태
  flushSync(() => {
    setFlag(f => !f);
  });
  // 이 과정이 끝났을 때 React는 DOM을 업데이트한 상태
}

// index.tsx

// v17 이전
ReactDOM.render(
  <React.StrictMode>
    <App />
  </React.StrictMode>,
  document.getElementById('root')
);

// v18 이후(createRoot)
ReactDOM.createRoot(document.getElementById("root") as HTMLDivElement).render(
  <React.StrictMode>
    <App />
  </React.StrictMode>
);

// App.tsx
// v17 이전
import { useState } from "react";
const [urgentCount, setUrgentCount] = useState<number>(0);
const handleUrgentChange = (e: React.ChangeEvent<HTMLInputElement>) => {
  console.log(urgentCount);
  setUrgentCount((prev) => e.target.value.length ** 6);
};

<button onClick={() => handleUpdateType("Urgent")}>
  Urgent updates
</button>

<>
  <label>Urgent : </label>
  <input type="text" onChange={handleUrgentChange} />
  <br />
  {[...Array(urgentCount)].map((n, index) => {
    return <img src={person} alt="" />;
  })}
</>

// v18 이후
import { useState, useTransition } from "react";
const [transitionCount, setTransitionCount] = useState<number>(0);
const [isPending, startTransition] = useTransition();

const handleTransitionChange = (
  e: React.ChangeEvent<HTMLInputElement>
): void => {
  startTransition(() => {
    console.log(transitionCount);
    setTransitionCount((prev) => e.target.value.length ** 6);
  });
};

<>
  <label>Transition : </label>
  <input type="text" onChange={handleTransitionChange} />
  <br />
  {[...Array(transitionCount)].map((n, index) => {
    return <img src={person} alt="" />;
  })}
</>

function Typeahead() {
  const query = useSearchQuery('');
  const deferredQuery = useDeferredValue(query);

  const deferredValue = useDeferredValue(value, {
    timeoutMs: 5000,
  });

  // Memoizing tells React to only re-render when deferredQuery changes,
  // not when query changes.
  const suggestions = useMemo(
    () => <SearchSuggestions query={deferredQuery} />,
    [deferredQuery],
  );

  return (
    <>
      <SearchInput query={query} />
      <Suspense fallback="Loading results...">{suggestions}</Suspense>
    </>
  );
}

import { renderToString } from 'react-dom';

renderToString(
  <Layout>
    <NavBar />
    <Sidebar />
    <RightPane>
      <Post />
      <Suspense fallback={<Spinner />}>
        <Comments />
      </Suspense>
    </RightPane>
  </Layout>,
);

import { pipeToNodeWritable } from 'react-dom/server';
// data fetching 오래 걸릴경우(Comments)
const { startWriting, abort } = renderToPipeableStream(
  <Layout>
    <NavBar />
    <Sidebar />
    <RightPane>
      <Post />
      <Suspense fallback={<Spinner />}>
        <Comments />
      </Suspense>
    </RightPane>
  </Layout>,
  {
    onShellReady() {
      // The content above all Suspense boundaries is ready.
      // If something errored before we started streaming, we set the error code appropriately.
      res.statusCode = didError ? 500 : 200;
      res.setHeader('Content-type', 'text/html');
      stream.pipe(res);
    },
    onShellError(error) {
      // Something errored before we could complete the shell so we emit an alternative shell.
      res.statusCode = 500;
      res.send(
        '<!doctype html><p>Loading...</p><script src="clientrender.js"></script>',
      );
    },
    onAllReady() {
      // If you don't want streaming, use this instead of onShellReady.
      // This will fire after the entire page content is ready.
      // You can use this for crawlers or static generation.
      // res.statusCode = didError ? 500 : 200;
      // res.setHeader('Content-type', 'text/html');
      // stream.pipe(res);
    },
    onError(err) {
      didError = true;
      console.error(err);
    },
  },
);

// 특정 컴포넌트 코드량이 커서 로딩이 오래 걸린경우
// 특정 컴포넌트의 로직이 복잡해서 시간이 오래 걸릴 경우
import React, { Suspense } from 'react';

const OtherComponent = React.lazy(() => import('./OtherComponent'));

function MyComponent() {
  return (
    <div>
      <Suspense fallback={<div>Loading...</div>}>
        <OtherComponent />
      </Suspense>
    </div>
  );
}

<Layout>
  <NavBar />
  <Suspense fallback={<Spinner />}>
    <Sidebar />
  </Suspense>
  <RightPane>
    <Post />
    <Suspense fallback={<Spinner />}>
      <Comments />
    </Suspense>
  </RightPane>
</Layout>

import { useSyncExternalStore } from 'react';

const useStore = (store, selector) => {
  return useSyncExternalStore(
    store.subscribe,
    useCallback(() => selector(store.getState(), [store, selector])),
  );
};