JSFiddle - React, Tailwind, and code Playground

HTML

<div class="blocks">
   <div class="block" id="block1">block1</div>
   <div class="block" id="block2">block2</div>
   <div class="block" id="block3">block3</div>
   <div class="block" id="block4">block4</div>
</div>

CSS

.active { font-weight: bold; }

JavaScript

$(function(){
    var $blocks = $('.blocks .block');
    var timeout = 1000;
    var index = 0;
    setInterval(function(){
        $blocks.removeClass('active');
        $blocks.eq(index++).addClass('active');
        if (index == $blocks.size()) {
            index = 0;
        }
    }, timeout);
});