Что то доделали. Надо сменить ветку

src/App.jsx

@@ -1,98 +1,96 @@
-import { useState, useRef, useEffect } from 'react'
+import React from 'react'
+import styles from './App.module.scss'
 
-function Example() {
-    const myRef = useRef(null);
-    const counterRef = useRef(0);
-    const [text, setText] = useState('');
-    
-    useEffect(() => {
-        myRef.current?.focus();
-    },[]);
-        
-    const handlerFocus = () => {
-        if(myRef.current) {
-            myRef.current.focus();
+class Clock extends React.Component {
+    constructor(props) {
+        super(props);
+        this.state = { date: new Date() };
     }
-    };
-    
-    const handlerChange = (event) => {
-        setText(event.target.value);
+    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>
     }
-    counterRef.current += 1;
-    return <div>
-        <input ref={myRef} text="text" /><br />
-        Количество рендров: {counterRef.current}<br />
-        <input text="text" onChange={handlerChange} value={text} />
-        <button onClick={handlerFocus}>Поставить фокус на поле</button>
-    </div>
 }
 
-function ClickCounter() {
-    const countRef = useRef(0);
+function Clock2() {
+    const [date, setDate] = React.useState(new Date());
     
-    function handlerClick() {
-        countRef.current += 1;
-        //alert(`Нажали ${countRef.current} раз.`);
-    }
-    return <button onClick={handlerClick}>Нажать</button>;
+    React.useEffect(()=>{
+        const timerId = setInterval(()=> setDate(new Date()),1000);
+        return () => clearInterval(timerId);
+    },[])
+    
+    return <div>{date.toLocaleTimeString()}</div>
 }
 
-function TimerComponent() {
-    const timerIdRef = useRef(null);
+function MouseTracker() {
+    const [x,setX] = React.useState(0);
+    const [y,setY] = React.useState(0);
     
-    useEffect(() => {
-        timerIdRef.current = setTimeout(() => {
-            alert("Прошло 3 секунды");
-        }, 3000);
+    const mouseMoveHandler = event => {
+        setX(event.clientX);
+        setY(event.clientY);
+    }
+    
+    React.useEffect(()=>{
+        document.addEventListener('mousemove', mouseMoveHandler);
         return () => {
-            clearTimeout(timerIdRef.current);
+            document.removeEventListener('mousemove',mouseMoveHandler);
         }
     },[]);
+    
+    return <div className={styles.container}>
+        <p>Координаты мыши:<br /> X:<b>{x}</b>, Y:<b>{y}</b></p>
+        </div>
 }
 
-function PrevValue({ value }) {
-    const prevValueRef = useRef();
+function DataFetcher() {
+    const [posts,setPosts] = React.useState([]);
+    const [users,setUsers] = React.useState([]);
     
-    useEffect(() => {
-        prevValueRef.current = value;
-    });
-    return (
-        <div>
-            Текущее значение: {value} <br />
-            Предыдущее значение: {prevValueRef.current}
-        </div>
-    )
-}
+    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));
         
-function ScrollList() {
-    const listRef = useRef(null);
-    
-    const scrollToBottom = () => {
-        if(listRef.current) {
-            listRef.current.scrollTop = listRef.current.scrollHeight;
-        }
     }
     
-    return (
-        <>
-            <div ref={listRef} 
-               style={{height: 200, overflowY: 'auto', border: '1px solid black'}}>
-                {[...Array(20)].map((_,i)=>(
-                    <div key={i}>Элемент № {i + 1}</div>
-                ))}
-            </div>
-            <button onClick={scrollToBottom}>Прокрутить вниз</button>
-        </>
-    )
+    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>
 }
 
 function App() {
-    
+    const [tracker,setTracker] = React.useState(false);
+    const [dataFetcher,setDataFetcher] = React.useState(false);
     
 	return (
 		<>
 			<h1>Hello, React!</h1>
-            <ScrollList />
+            <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>
 )