index.html
<!DOCTYPE html>
<html lang="ja">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>My JavaScript</title>
</head>
<body>
<script src="main.js"></script>
</body>
</html>main.js
'use strict';
{
// let num = 10;
// const numBackup = num;
// num = 99;
// console.log(num);
// console.log(numBackup);
const nums = [10, 20, 30];
const numBackup = nums;
nums[0] = 99;
console.log(nums);
console.log(numBackup);
}