JSFiddle - React, Tailwind, and code Playground

HTML

<div id="cart">
( <span id="cart-span">0</span> )
</div>
<a id="add" href="#">Добавить товар</a>

CSS

#cart{
  border:solid 10px red;
  font-weight:700;
  padding:10px;
  text-align:center;
}

JavaScript

$("#add").on("click", function(){
	var current_value = parseInt($("#cart-span").html());
  var new_value = current_value + 1;
  $("#cart-span").html( new_value );
})