当我尝试向其中添加一个类时，Console.log告诉我我的div为null

Question

我添加了一个keydown事件监听器，它只会让我的boxdiv显示出来。然而，当我按下一个键时，什么也没发生，而是控制台告诉我，"；未捕获类型错误：框为空；。不知道问题出在哪里，请帮助

这是我的App.js:

import "./styles.css"
const box = document.querySelector(".box");
const notification = document.querySelector(".notif");
window.onload = document.addEventListener("keydown", e => {
box.classList.add("active");
notification.classList.add("inactive");
})
function App() {
return (
<>
<div className='notif'>Press Any Key</div>
<div className='box'>
<h2>20</h2>
<h1>Shift</h1>
</div>
</>
);
}
export default App;

和css：@导入url('https://fonts.googleapis.com/css2?family=Poppins&display=swap'(；

:root {
--teal: #00ccff;
--pink: #d400d4;
}
*, *::before, *::after {
margin: 0;
padding: 0;
box-sizing: border-box;
font-family: 'Poppins', sans-serif;
}
body {
display: flex;
min-height: 100vh;
align-items: center;
justify-content: center;
background: #0e1538;
}
.box {
position: relative;
width: 300px;
height: 300px;
display: flex;
justify-content: center;
align-items: center;
background: rgba(0,0,0,.5);
overflow: hidden;
border-radius: 20px;
flex-direction: column;
display: none;
}
.box.active {
display: flex;
}
.box::before {
content: '';
position: absolute;
width: 150px;
height: 140%;
background: linear-gradient(var(--teal), var(--pink));
animation: animate 4s linear infinite;
}
@keyframes animate {
0%{
transform: rotate(0deg);
}
100% {
transform: rotate(360deg);
}
}
.box::after {
content: '';
position: absolute;
background: #0e1538;
inset: 4px;
border-radius: 16px;
}
.box h2 {
position: relative;
color: white;
font-size: 5em;
z-index: 10;
}
.box h1 {
position: relative;
color: white;
z-index: 10;
background: rgba(0,0,0,.5);
width: 97.5%;
text-align: center;
letter-spacing: 5px;
}
.notif {
font-size: 3em;
background: var(--teal);
padding: 0 20px;
border-radius: 20px;
}
.notif.inactive {
display: none;
}

Answer 1

按键时，您应该使用新的类信息来维护/更新状态。在useEffect中添加侦听器(以及一个清理函数(。当按下某个键时，监听器会调用一个更新状态的函数(在这种情况下，切换活动/非活动类(。该组件被重新呈现，并且元素的类名使用一个小助手函数来更改。

const { useEffect, useState } = React;
function Example() {
// Initialise state
const [ active, setActive ] = useState({
box: false,
notification: true
});
// A function to update the state
function toggleActive() {
setActive(prev => ({
box: !prev.box,
notification: !prev.notification
}));
}

// An effect that adds a listener, w/ a cleanup function
useEffect(() => {
window.addEventListener('keydown', toggleActive);
return () => {
window.removeEventListener('keydown', toggleActive);
}
}, []);
// Returns the active status of the element from state
function isActive(type) {
return active[type] ? 'active' : 'inactive';
}
return (
<div>
<div className={isActive('notification')}>
Press Any Key
</div>
<div className={isActive('box')}>
<h2>20</h2>
<h1>Shift</h1>
</div>
</div>
);
}
ReactDOM.render(
<Example />,
document.getElementById('react')
);

.active { color: red; }
.inactive { color: black; }

<script src="https://cdnjs.cloudflare.com/ajax/libs/react/17.0.2/umd/react.production.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/17.0.2/umd/react-dom.production.min.js"></script>
<div id="react"></div>

Answer 2

import "./styles.css"
import {useEffect} from 'react'

function App() {
useEffect(()=>{
const box = document.querySelector(".box");
const notification = document.querySelector(".notif");
document.addEventListener("keydown", e => {
box.classList.add("active");
notification.classList.add("inactive");
})
},[])
return (
<>
<div className='notif'>Press Any Key</div>
<div className='box'>
<h2>20</h2>
<h1>Shift</h1>
</div>
</>
);
}
export default App;