Untitled

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>object</title>
</head>
<body>
	<div class="wrap"></div>
	<div class="wrap"></div>
	<div class="wrap"></div>
	<div class="wrap"></div>
	<div class="wrap"></div>
	<div class="wrap"></div>
	<div class="wrap"></div>
	<div class="wrap"></div>
	<div class="wrap"></div>
	<div class="wrap"></div>
<script>
	let box = document.querySelectorAll(".wrap");

	for (let i = 0; i < box.length; i++) {

		if (i % 2 == 0) {
			box[i].style.width = '30px';
			box[i].style.height = '30px';
			box[i].style.backgroundColor = 'red';
		} else {
			console.log('error');
		}

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