Untitled

// index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>

<body>
</body>

<script src="index.js"></script>
</html>

// index.js
'use strict';

function toggleClass(elem, className) {
    let classes = elem.className.split(' ');

    if (classes.includes(className)) {
        classes.splice(classes.indexOf(className), 1)
    } else {
        classes.push(className);
    }

    classes = classes.join(' ')

    console.log("\nClasses: ", classes)
    console.log("Elem before:", elem);
    elem.setAttribute("class", classes);
    console.log("Elem after:", elem);

    return elem
}

let $div = document.createElement('div');
$div.className = "box shadow radius";

$div = toggleClass($div, 'border')

$div = toggleClass($div, 'border')