Black_Star,
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport"
content="width=device-width, user-scalable=no, initial-scale=1.0, maximum-scale=1.0, minimum-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
</head>
<body>
<style>
.block{
width: 50px;
height: 50px;
background-color: #f00;
}
.testBlock{
width: 100px;
height: 100px;
background-color: #f0f;
}
</style>
<div class="block one two"></div>
<div class="testBlock"></div>
<script>
var allClasses = $(".block").prop("className");
$('<div class="testBlock"></div>').addClass(allClasses).appendTo("body")
</script>
</body>
</html>