useMemo

package.json

@@ -4,13 +4,12 @@
   "version": "0.0.0",
   "type": "module",
   "scripts": {
-    "dev": "vite",
+    "dev": "vite", 
     "build": "vite build",
     "lint": "eslint .",
     "preview": "vite preview"
   },
   "dependencies": {
-    "info": "^1.0.0",
     "react": "^19.1.0",
     "react-dom": "^19.1.0"
   },
@@ -23,8 +22,6 @@
     "eslint-plugin-react-hooks": "^5.2.0",
     "eslint-plugin-react-refresh": "^0.4.19",
     "globals": "^16.0.0",
-    "sass": "^1.99.0",
-    "sass-embedded": "^1.99.0",
     "vite": "^6.3.5"
   }
 }

src/App.jsx

@@ -1,29 +1,82 @@
-import styles from './styles/App.module.scss'
-import React, { useState, useCallback } from 'react'
+import memo, { useMemo, useCallback, useState } from 'react'
 
-function CounterButton({label, onClick}) {
+function MyComponent() {
+    const [count, setCount] = useState(0);
+    const [name, setName] = useState('John');
+    
+    const greeting = useMemo(() => `Hello, ${name}!`,[name]);
+    
     return (
-        <button onClick={onClick} className={styles.counter_button}>{label}</button>
+        <>
+        <h2>{greeting}</h2>
+        <button onClick={()=>setName('Alex')}>New name</button>
+        </>
     )
 }
-function CounterDisplay({value}) {
-    return (<div className={styles.counter_display}>
-            Значение счетчика: <b>{value}</b>
-    </div>)
+
+function MyComponent2() {
+    const [data,setData] = useState([{theme:'light'}])
+    const userPreferences = useMemo(()=>({theme: 'dark', language: 'en'}),[]);
+    
+    const processData = useMemo(()=>{
+        return data.map(item => ({ ...item,theme: userPreferences.theme }))
+    },[data, userPreferences]);
+    console.log(processData);
+    return (
+        <>
+            <h2 className={data.theme}>2</h2>
+            <button onClick={()=>setData(data)}>set new theme</button>
+        </>
+    )
+}
+
+function MyComponent3() {
+    const sum = useMemo(()=> a+b,[a,b]);//Бессмысленное применение и вредное
+
+    //Применение оправдано
+    const complexResult = useMemo(()=>{
+        return heavyMethOperations(largeDataSet);
+    },[largeDataSet])
+}
+
+function ParentComponent({ items }) {
+    const [filter, setFilter] = useState('');
+    
+    const listProps = useMemo(()=>({
+            items: items.filter(item => item.name.includes(filter)),
+            onItemClick: id => console.log('Clicked',id),
+            theme: 'dark'
+        }),[items,filter]);
+    
+    return (
+        <>
+        <Memoized {...listProps} />
+        <button onClick={()=>setFilter('Bob')}>Click</button>
+        </>
+    )
+}
+
+function Memoized(props) {
+    console.log(props);
+    return (
+        <>
+        <h3 className={props.theme}>Memoized</h3>
+        {props.items.map((item) => (
+         <>
+            <li>
+                {item.name}
+            </li>
+         </>))}
+        </>
+    )
 }
 
 function App() {
-    const [counter,setCounter] = useState(0);
-    
-    const increment = () => setCounter(prev => prev + 1);
-    const decrement = () => setCounter(prev => prev - 1);
-    
 	return (
 		<>
-            <h2>Практика по созданию счетчика с колбэками</h2>
-            <CounterButton label="Увеличить" onClick={increment} />
-            <CounterButton label="уменьшить" onClick={decrement} />
-            <CounterDisplay value={counter} />
+			{/* Добавьте тестовый контент для проверки */}
+			<h1>Hello, React!</h1>
+            <ParentComponent items={[{name:'Alex'},{name:'Bob'}]} />
 		</>
 	)
 }

src/main.jsx

@@ -1,10 +1,8 @@
 import { StrictMode } from 'react'
 import { createRoot } from 'react-dom/client'
-
+// удаляем import './index.css'
 import App from './App.jsx'
 
 createRoot(document.getElementById('root')).render(
-	<StrictMode>
 		<App />
-	</StrictMode>
 )

src/styles/App.module.scss

@@ -1,22 +0,0 @@
-.container {
-    display:flex;
-    flex-direction: row;
-    border: 1px solid black;
-    border-radius: 5px;
-    margin-bottom: 10px;
-    p {
-        padding: 5px 10px;
-    }
-}
-.counter {
-    &_display{
-        border: 1px solid black;
-        border-width: 3px;
-        border-radius: 10px;
-        padding: 10px;
-        width: 300px;
-    }
-    &_button{
-        padding: 10px;
-    }
-}