Разобрали useRef

src/App.jsx

@@ -1,50 +1,98 @@
-import { useState, useReducer } from 'react'
+import { useState, useRef, useEffect } from 'react'
 
-function counterReducer(state, action) {
+function Example() {
-    switch (action.type) {
+    const myRef = useRef(null);
-        case 'increment':
+    const counterRef = useRef(0);
-            return state + 1
+    const [text, setText] = useState('');
-        case 'decrement':
+    
-            return state - 1
+    useEffect(() => {
-        case 'reset':
+        myRef.current?.focus();
-            return 0
+    },[]);
-        case 'set_value':
+        
-            return action.value
+    const handlerFocus = () => {
-        default:
+        if(myRef.current) {
-            return state
+            myRef.current.focus();
         }
+    };
+    
+    const handlerChange = (event) => {
+        setText(event.target.value);
+    }
+    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 Counter() {
+function ClickCounter() {
-    const [count, dispatch] = useReducer(counterReducer,0);
+    const countRef = useRef(0);
     
+    function handlerClick() {
+        countRef.current += 1;
+        //alert(`Нажали ${countRef.current} раз.`);
+    }
+    return <button onClick={handlerClick}>Нажать</button>;
+}
+
+function TimerComponent() {
+    const timerIdRef = useRef(null);
+    
+    useEffect(() => {
+        timerIdRef.current = setTimeout(() => {
+            alert("Прошло 3 секунды");
+        }, 3000);
+        return () => {
+            clearTimeout(timerIdRef.current);
+        }
+    },[]);
+}
+
+function PrevValue({ value }) {
+    const prevValueRef = useRef();
+    
+    useEffect(() => {
+        prevValueRef.current = value;
+    });
     return (
         <div>
-            <span>Счетчик: {count}</span>
+            Текущее значение: {value} <br />
-            <button onClick={()=> dispatch({type: 'increment'})}>+</button>
+            Предыдущее значение: {prevValueRef.current}
-            <button onClick={()=> dispatch({type: 'decrement'})}>-</button>
-            <button onClick={()=> dispatch({type: 'reset'})}>reset</button>
-            <button onClick={()=> dispatch({type: 'set_value', value: 5})}>set 5</button>
         </div>
     )
 }
 
-function App() {
+function ScrollList() {
-    const [tasks, setTasks] = useState([])
+    const listRef = useRef(null);
-    const [filter, setFilter] = useState('all')
-    const [sortBy,setSortBy] = useState('date')
     
-    const addTask = newTask => {
+    const scrollToBottom = () => {
-        setTasks([...tasks,newTask])
+        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>
+        </>
+    )
+}
+        
+function App() {
     
     
 	return (
 		<>
-			{/* Добавьте тестовый контент для проверки */}
 			<h1>Hello, React!</h1>
-            <Counter />
+            <ScrollList />
 		</>
 	)
 }

src/App.module.scss

@@ -1,15 +0,0 @@
-.container {
-/*    display: flex;
-    flex-direction: column;
-    flex-wrap: nowrap;*/
-    height: 500px;
-    overflow: auto;
-    border: 1px solid black;
-}
-.item {
-    font-size: 16pt;
-    border: 1px dotted gray;
-    /*margin: 5px 10px;
-    padding: 10px;*/
-    border-radius: 5px;
-}

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>
 )