그리고 자바랑 CSS랑은 달라서, querySelectorAll('a') 로 선택을 하시면 안됩ㄴ비다.
querySelectorAll('a')는 a태그가 달린 모든 걸 선택한다는게 아니고, 모든걸 배열로 가져온다는 뜻입니다.
배열은 나중에 나오겠지만,
쉽게말하면 querySelectorAll('a')[0] 이 첫번째 a태그, querySelectorAll('a')[1] 이 두번째 a태그... 이런식으로 지정이 되는거라, for문으로 처음부터 끝까지 돌면서 변경해줘야하는것 같아요.
<!DOCTYPE html>
<html>
<head>
<title> WEB1 - JavaScript</title>
<meta charset="utf-8">
</head>
<body>
<h1><a href="index.html">WEB</a></h1>
<input id="night_day" type="button" value="night" onclick="
if(document.querySelector('#night_day').value === 'night'){
document.querySelector('body').style.backgroundColor = 'black';
document.querySelector('body').style.color = 'white';
document.querySelector('#night_day').value = 'day';
} else {
document.querySelector('body').style.backgroundColor = 'white';
document.querySelector('body').style.color = 'black';
document.querySelector('#night_day').value = 'night';
}
">
<ol>
<li><a href="1.html">HTML</a></li>
<li><a href="2.html">CSS</a></li>
<li><a href="3.html">JavaScript</a></li>
</ol>
<h2>JavaScript</h2>
<p>
JavaScript (/ˈdʒɑːvəˌskrɪpt/[6]), often abbreviated as JS, is a high-level, dynamic, weakly typed, prototype-based, multi-paradigm, and interpreted programming language. Alongside HTML and CSS, JavaScript is one of the three core technologies of World Wide Web content production. It is used to make webpages interactive and provide online programs, including video games. The majority of websites employ it, and all modern web browsers support it without the need for plug-ins by means of a built-in JavaScript engine. Each of the many JavaScript engines represent a different implementation of JavaScript, all based on the ECMAScript specification, with some engines not supporting the spec fully, and with many engines supporting additional features beyond ECMA.
</p>