Сделали React.memo и React.useMemo

src/App.jsx

@@ -1,97 +1,60 @@
-import React from 'react'
-import styles from './App.module.scss'
+import { memo, useState, useMemo } from 'react'
 
-class Clock extends React.Component {
-    constructor(props) {
-        super(props);
-        this.state = { date: new Date() };
-    }
-    componentDidMount() {
-        this.timerId = setInterval( () => this.tick(), 1000);
-    }
-    componentWillUnmount() {
-        clearInterval(this.timerId)
-    }
-    tick() {
-        this.setState({ date: new Date() })
-    }
-    render() {
-        return <div>{this.state.date.toLocaleTimeString()}</div>
-    }
-}
-
-function Clock2() {
-    const [date, setDate] = React.useState(new Date());
-    
-    React.useEffect(()=>{
-        const timerId = setInterval(()=> setDate(new Date()),1000);
-        return () => clearInterval(timerId);
-    },[])
-    
-    return <div>{date.toLocaleTimeString()}</div>
-}
-
-function MouseTracker() {
-    const [x,setX] = React.useState(0);
-    const [y,setY] = React.useState(0);
-    
-    const mouseMoveHandler = event => {
-        setX(event.clientX);
-        setY(event.clientY);
-    }
-    
-    React.useEffect(()=>{
-        document.addEventListener('mousemove', mouseMoveHandler);
-        return () => {
-            document.removeEventListener('mousemove',mouseMoveHandler);
+function memoize(fn) {
+    const cache = new Map();
+    return function(...args) {
+        const key = JSON.stringify(args);
+        if (cache.has(key)) {
+            return cache.get(key);
         }
-    },[]);
-    
-    return <div className={styles.container}>
-        <p>Координаты мыши:<br /> X:<b>{x}</b>, Y:<b>{y}</b></p>
-        </div>
+        const result = fn.apply(this, args);
+        cache.set(key,result);
+        return result;
+    }
 }
 
-function DataFetcher() {
-    const [posts,setPosts] = React.useState([]);
-    const [users,setUsers] = React.useState([]);
+function fibonachi(n, memo={}) {
+    if (n in memo) {console.log(n);return memo[n];}
+    if (n <= 1) return 1;
+    memo[n] = fibonachi(n-1, memo) + fibonachi(n - 2, memo);
+    return memo[n];
+}
+
+const MyComponent = ({ value }) => {
+    console.log('Rendring');
+    return <div>{value}</div>
+}
+
+const MemoizedComponent = memo(MyComponent);
+
+const Child = memo(({ children }) => {
+    console.log('Child component rendred');
+    return <div>{children}</div>;
+});
+
+const Parent = () => {
+    const [count,setCount] = useState(0);
+    const [text, setText] = useState('');
     
-    const getPosts = async () => {
-        const response = await fetch('https://jsonplaceholder.typicode.com/posts');
-        const data = await response.json();
-        setUsers(data.reduce((acc, value) => {
-            if (!(value.userId in acc))
-                return [...acc,value.userId]
-            return acc
-        }))
-        console.log(users);
-        setPosts(data.slice(1,10));
-        
-    }
+    const stableChild = useMemo(() => <span>Memo text</span>,[]);
     
-    React.useEffect(() => {
-        console.log("Компонент создан");
-        getPosts();
-        return () => {console.log("Компонент удален");setPosts([])};
-    },[])
-    
-    return <div><ol>{posts.map(post => <li id={post.id}>{post.title}</li>)}</ol></div>
+    return (
+        <>
+            <Child>
+                {stableChild}
+            </Child>
+            <input type="text" onChange={(e) => setText(e.target.value)} value={text} />
+            <button onClick={() => setCount(count + 1)}>Increment {count}</button>
+        </>
+    )
 }
 
 function App() {
-    const [tracker,setTracker] = React.useState(false);
-    const [dataFetcher,setDataFetcher] = React.useState(false);
-    
 	return (
 		<>
 			<h1>Hello, React!</h1>
-            <Clock />
-            <Clock2 />
-            <button onClick={() => setTracker(!tracker)}>Показать/Скрыть трекер</button>
-            { tracker && <MouseTracker /> }
-            <button onClick={() => setDataFetcher(!dataFetcher)}>Показать/Скрыть посты</button>
-            { dataFetcher && <DataFetcher /> }
-		</>
+            <Parent />
+        </>
 	)
 }
 

src/App.module.scss

@@ -1,11 +0,0 @@
-.container {
-    width: 300px;
-    height: 200px;
-    border: 2px dashed grey;
-    margin: 10px;
-    text-align: center;
-    p {
-        margin-top: 20%;
-    }
-}
-

src/main.jsx

@@ -4,7 +4,5 @@ import { createRoot } from 'react-dom/client'
 import App from './App.jsx'
 
 createRoot(document.getElementById('root')).render(
-	<StrictMode>
 		<App />
-	</StrictMode>
 )