Сделали React.memo и React.useMemo

src/App.jsx

@@ -1,99 +1,60 @@
-import { useState, useRef, useEffect } from 'react'
+import { memo, useState, useMemo } from 'react'
 
-function Example() {
-    const myRef = useRef(null);
-    const counterRef = useRef(0);
+function memoize(fn) {
+    const cache = new Map();
+    return function(...args) {
+        const key = JSON.stringify(args);
+        if (cache.has(key)) {
+            return cache.get(key);
+        }
+        const result = fn.apply(this, args);
+        cache.set(key,result);
+        return result;
+    }
+}
+
+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('');
     
-    useEffect(() => {
-        myRef.current?.focus();
-    },[]);
-        
-    const handlerFocus = () => {
-        if(myRef.current) {
-            myRef.current.focus();
-        }
-    };
+    const stableChild = useMemo(() => <span>Memo text</span>,[]);
     
-    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 ClickCounter() {
-    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>
-            Текущее значение: {value} <br />
-            Предыдущее значение: {prevValueRef.current}
-        </div>
-    )
-}
-
-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>
+            <Child>
+                {stableChild}
+            </Child>
+            <input type="text" onChange={(e) => setText(e.target.value)} value={text} />
+            <button onClick={() => setCount(count + 1)}>Increment {count}</button>
         </>
     )
 }
-        
+
 function App() {
-    
-    
 	return (
 		<>
 			<h1>Hello, React!</h1>
-            <ScrollList />
-		</>
+            <Parent />
+        </>
 	)
 }