Biotoxsin,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.4.1/jquery.min.js"></script>
<script>
$(function() {
$.fn.plugin = function() {
alert($('.test')[0].offsetWidth);
alert(this[0].offsetWidth);
}
$('.test').plugin()
});
</script>
</head>
<body class="test">
</body>
</html>