JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<button class="btn" data-price="4.99" >
ADD 4.99
</button>

JavaScript

localStorage.setItem('totalPrice', '15.00');

$(document).on('click','.btn',function(e){

var price = $(this).attr('data-price');


var curPrice = localStorage.getItem('totalPrice');

var setPrice = Number(price) + Number(curPrice);

alert(setPrice);

});