In Shopify, real-time cart drawer updates are important for delivering a fast, seamless cart experience by:

  • Automatically reflecting changes in the cart drawer (additions, removals, quantity edits) without page reloads.
  • Providing instant feedback to users through real-time updates.
  • Maintaining cart state consistency and reducing friction in the shopping journey.

Why Is This Important?

  • Meet Modern UX Standards: Users expect cart changes to feel instant and intuitive, without having to reload or navigate away from the current view.
  • Reduce Context Switching: Real-time updates keep users engaged with the product experience, reducing drop-off between browsing and checkout.
  • Ensure Cart Accuracy: Outdated carts or delayed cart processing can result in wrong calculation of total items. This can lead to confusion about product quantity, or accidental double purchases.
  • Improve Mobile Experience: On mobile, drawer-based carts with real-time updates significantly enhance speed and usability compared to full page reloads.

Implementation Plan

1. Enable Real-Time Updates with AJAX

Leverage asynchronous requests to Shopify’s /cart.js endpoint or your backend’s cart API.

Example:

js
fetch('/cart.js') .then(response => response.json()) .then(cart => renderCartDrawer(cart));
  • Trigger this call after actions like adding, removing, or updating a line item.
  • Avoid refreshing the entire page just fetch the cart state and update relevant DOM elements.

2. Modularize the Cart Rendering Logic

Use a centralized function (e.g., renderCartDrawer(cart)) to:

  • Loop through cart.items and rebuild the drawer UI.
  • Update subtotals, line item quantities, and promotional info.

Example:

js
function renderCartDrawer(cart) { const cartContainer = document.querySelector('.cart-drawer-items'); cartContainer.innerHTML = ''; cart.items.forEach(item => { const itemHTML = ` <div class="cart-item"> <img src="${item.image}" alt="${item.title}" /> <div class="cart-details"> <p>${item.title}</p> <p>Qty: ${item.quantity}</p> <p>$${(item.final_line_price / 100).toFixed(2)}</p> </div> </div>`; cartContainer.insertAdjacentHTML('beforeend', itemHTML); }); document.querySelector('.cart-subtotal').textContent = `$${(cart.total_price / 100).toFixed(2)}`;
}

3. Handle Add/Remove/Update Actions

Tie cart interactions to events that trigger the fetch-update cycle:

Add to Cart

js
fetch('/cart/add.js', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ id: variantId, quantity: 1 })
}).then(() => refreshCartDrawer());

Remove from Cart

js
fetch('/cart/change.js', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ id: lineItemKey, quantity: 0 })
}).then(() => refreshCartDrawer());

Update Quantity

js
fetch('/cart/change.js', { method: 'POST', headers: { 'Content-Type': 'application/json' }, body: JSON.stringify({ id: lineItemKey, quantity: newQty })
}).then(() => refreshCartDrawer());

Common refresher:

js
function refreshCartDrawer() { fetch('/cart.js') .then(res => res.json()) .then(cart => renderCartDrawer(cart));
}

4. Display Loading or Transitional Feedback

To improve UX during async operations, show a spinner or skeleton loader in the cart drawer:

js
function showCartLoading() { document.querySelector('.cart-drawer').classList.add('loading');
}

Use this between request start and renderCartDrawer completion.

5. Preserve Scroll Position and State

Avoid collapsing the cart drawer or resetting scroll positions on every update.

  • Instead, diff and patch only updated DOM elements (if needed).
  • Maintain focus on quantity fields or remove buttons post-update.

Backend/API Considerations

  • Ensure your backend supports cart APIs with consistent and reliable responses (Shopify’s /cart.js or custom equivalent).
  • Confirm support for concurrent requests or race-condition management during rapid cart interactions.
  • For headless setups, implement cart session persistence and idempotent update endpoints.