Untitled

<!DOCTYPE html>
<html lang="ja">
<head>
	<meta charset="UTF-8">
	<title>get h3 test</title>
	<style>
		h3:before{
			content: attr(data-index)"";
			display: block;
		}
	</style>
</head>
<body>


<div>
	<h3 data-index="">test1</h3>
	<p>test</p>
	<h3 data-index="">test2</h3>
	<p>test</p>
	<h3 data-index="">test3</h3>
	<p>test</p>
</div>

<script>
	const getH3 = ()=>{
		let num;
		let i;
		const allH3 = document.querySelectorAll("h3");
		for(i = 0; i <= allH3.length; i++ ){
			num = allH3[i].dataset;
			num.index = Number([i])+1;
		}
	};
	getH3();

</script>
</body>
</html>