Cart.js is a very small open source Javascript library that makes the addition of powerful Ajax cart functionality to your Shopify theme a breeze.

It's designed to be simple to use, while providing some really powerful and nifty features, like:

<ul class="check-list">
    <li><strong>Simple, consistent API</strong> for cart manipulation;</li>
    <li><strong>Data API</strong> for markup-only use without needing to write a line of Javascript;</li>
    <li><strong>DOM Binding</strong> to dynamically render HTML templates as your cart changes.</li>
</ul>

You don't need to worry about ensuring your Ajax requests are synchronous, binding event listeners, or updating the DOM.