Что то доделали. Надо сменить ветку

src/App.jsx

@@ -1,82 +1,96 @@
-import memo, { useMemo, useCallback, useState } from 'react'
+import React from 'react'
+import styles from './App.module.scss'
 
-function MyComponent() {
+class Clock extends React.Component {
-    const [count, setCount] = useState(0);
+    constructor(props) {
-    const [name, setName] = useState('John');
+        super(props);
-    
+        this.state = { date: new Date() };
-    const greeting = useMemo(() => `Hello, ${name}!`,[name]);
+    }
-    
+    componentDidMount() {
-    return (
+        this.timerId = setInterval( () => this.tick(), 1000);
-        <>
+    }
-        <h2>{greeting}</h2>
+    componentWillUnmount() {
-        <button onClick={()=>setName('Alex')}>New name</button>
+        clearInterval(this.timerId)
-        </>
+    }
-    )
+    tick() {
+        this.setState({ date: new Date() })
+    }
+    render() {
+        return <div>{this.state.date.toLocaleTimeString()}</div>
+    }
 }
 
-function MyComponent2() {
+function Clock2() {
-    const [data,setData] = useState([{theme:'light'}])
+    const [date, setDate] = React.useState(new Date());
-    const userPreferences = useMemo(()=>({theme: 'dark', language: 'en'}),[]);
     
-    const processData = useMemo(()=>{
+    React.useEffect(()=>{
-        return data.map(item => ({ ...item,theme: userPreferences.theme }))
+        const timerId = setInterval(()=> setDate(new Date()),1000);
-    },[data, userPreferences]);
+        return () => clearInterval(timerId);
-    console.log(processData);
+    },[])
-    return (
+    
-        <>
+    return <div>{date.toLocaleTimeString()}</div>
-            <h2 className={data.theme}>2</h2>
-            <button onClick={()=>setData(data)}>set new theme</button>
-        </>
-    )
 }
 
-function MyComponent3() {
+function MouseTracker() {
-    const sum = useMemo(()=> a+b,[a,b]);//Бессмысленное применение и вредное
+    const [x,setX] = React.useState(0);
-
+    const [y,setY] = React.useState(0);
-    //Применение оправдано
+    
-    const complexResult = useMemo(()=>{
+    const mouseMoveHandler = event => {
-        return heavyMethOperations(largeDataSet);
+        setX(event.clientX);
-    },[largeDataSet])
+        setY(event.clientY);
+    }
+    
+    React.useEffect(()=>{
+        document.addEventListener('mousemove', mouseMoveHandler);
+        return () => {
+            document.removeEventListener('mousemove',mouseMoveHandler);
+        }
+    },[]);
+    
+    return <div className={styles.container}>
+        <p>Координаты мыши:<br /> X:<b>{x}</b>, Y:<b>{y}</b></p>
+        </div>
 }
 
-function ParentComponent({ items }) {
+function DataFetcher() {
-    const [filter, setFilter] = useState('');
+    const [posts,setPosts] = React.useState([]);
+    const [users,setUsers] = React.useState([]);
     
-    const listProps = useMemo(()=>({
+    const getPosts = async () => {
-            items: items.filter(item => item.name.includes(filter)),
+        const response = await fetch('https://jsonplaceholder.typicode.com/posts');
-            onItemClick: id => console.log('Clicked',id),
+        const data = await response.json();
-            theme: 'dark'
+        setUsers(data.reduce((acc, value) => {
-        }),[items,filter]);
+            if (!(value.userId in acc))
+                return [...acc,value.userId]
+            return acc
+        }))
+        console.log(users);
+        setPosts(data.slice(1,10));
+        
+    }
     
-    return (
+    React.useEffect(() => {
-        <>
+        console.log("Компонент создан");
-        <Memoized {...listProps} />
+        getPosts();
-        <button onClick={()=>setFilter('Bob')}>Click</button>
+        return () => {console.log("Компонент удален");setPosts([])};
-        </>
+    },[])
-    )
+    
-}
+    return <div><ol>{posts.map(post => <li id={post.id}>{post.title}</li>)}</ol></div>
-
-function Memoized(props) {
-    console.log(props);
-    return (
-        <>
-        <h3 className={props.theme}>Memoized</h3>
-        {props.items.map((item) => (
-         <>
-            <li>
-                {item.name}
-            </li>
-         </>))}
-        </>
-    )
 }
 
 function App() {
+    const [tracker,setTracker] = React.useState(false);
+    const [dataFetcher,setDataFetcher] = React.useState(false);
+    
 	return (
 		<>
-			{/* Добавьте тестовый контент для проверки */}
 			<h1>Hello, React!</h1>
-            <ParentComponent items={[{name:'Alex'},{name:'Bob'}]} />
+            <Clock />
+            <Clock2 />
+            <button onClick={() => setTracker(!tracker)}>Показать/Скрыть трекер</button>
+            { tracker && <MouseTracker /> }
+            <button onClick={() => setDataFetcher(!dataFetcher)}>Показать/Скрыть посты</button>
+            { dataFetcher && <DataFetcher /> }
 		</>
 	)
 }

src/App.module.scss

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

src/main.jsx

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