JSFiddle - React, Tailwind, and code Playground

HTML

<div>
<div class="cat"></div>
<div class="cat"></div>
<div class="cat"></div>
<div class="cat"></div>
<div class="cat"></div>
<div class="cat"></div>
<div class="cat"></div>
</div>

CSS

* {
	margin: 0;padding:0;
}
body {
	padding:30px;
}
.cat {
	display:none; /* provisoire en attendant le JS */
	width:20px;
	height:20px;
	border-radius:10px;
	background-color:lightblue;
}

JavaScript

idx=0;
function fadeEq(idx){
    $('.cat').eq(idx).fadeIn( 500, function() { if(++idx<7){fadeEq(idx);} }) 
}

fadeEq(0);