JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li>testing 1</li>
    <li>testing 2</li>
    <li>testing 3</li>
    <li>testing 4</li>
</ul>

CSS

.bold {
    font-weight:bold;
}

JavaScript

$('li').live('mouseover', function() {
    var t = this;
    $('li').each(function() {
        if (this != t) $(this).removeClass('bold');
        else $(this).addClass('bold');
    });
});