Shopify Functions vs Checkout UI Extensions: Which One Actually Does What

These two get confused constantly because they shipped around the same time and both touch checkout. They solve different problems, and picking the wrong one means rebuilding later.

⚙️ What Functions Actually Do

Shopify Functions run server-side logic that changes the math — discounts, shipping rates, payment method availability, validation. They don't render anything visually; they decide what the checkout calculates.

🎨 What Checkout UI Extensions Actually Do

Extensions render visual elements inside checkout — a banner, a custom field, an upsell block. They don't change pricing math on their own; they display and collect information within Shopify's checkout UI.

🔗 Why You Often Need Both

A "spend $50, get a free gift" feature needs a Function to actually apply the discount and a UI Extension to show the progress bar telling the customer how close they are. Picking only one gets you half the feature.

---

Originally published on the KLYX blog.