JSFiddle - React, Tailwind, and code Playground

HTML

<span>Monty Burns</span><br />
<span>Bart Simpson</span>

JavaScript

var spans = $('span');
spans.each(function(index, element) {
    var span = $(element);
    var spanText = span.text();
    var spanTextArray = spanText.split(' ');
    var spanTextArrayLength = spanTextArray.length;
    var lastName = spanTextArray[spanTextArrayLength -1];
    spanTextArray.pop();
    var firstName = spanTextArray.join(' ');
    span.text(firstName);
    var spanLastName = $('<span/>');
    spanLastName.css('font-weight', 'bold');
    spanLastName.css('margin-left', '5px');
    spanLastName.appendTo(span);
    spanLastName.text(lastName);
});