Here is how you add a class to an element such as a div.
Step 1 Create the div element using create.document() method
Step 2. Use the classlist.add() method to add the class or classes to the div
Step 3. Use appendChild() method to add div element to parent div
HTML
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>wecode101 add class when creating div element</title>
<link href="https://cdn.jsdelivr.net/npm/bootstrap@5.0.2/dist/css/bootstrap.min.css" rel="stylesheet" integrity="sha384-EVSTQN3/azprG1Anm3QDgpJLIm9Nao0Yz1ztcQTwFspd3yD65VohhpuuCOmLASjC" crossorigin="anonymous">
</head>
<body>
<main>
<!--Main layout-->
<div class="container" id="ctn"></div>
</main>
</body>
</html>
JS
const div = document.createElement('div'); //Create div element
div.classList.add('col-9', 'mx-auto', 'fs-2'); //Add classes to element
div.textContent = 'The Title'; //Add text element
const container = document.getElementById('ctn'); //Identify element to add div
container.appendChild(div); // Add div to to element