shopify-checkout-extensions

Create a Checkout UI extension with the Shopify CLI, register it against the purchase.checkout.block.render target, and use useApplyCartLinesChange to add the upsell variant. Merchants can then place the block anywhere via the checkout editor.

Write a run() function that sums cart line quantities, maps them to discount tiers, and returns a FunctionRunResult with a discounts array and discountApplicationStrategy. Compile it to WebAssembly and activate it from Admin → Discounts after deploying.

No, Shopify Functions cannot perform network I/O and must execute within a strict time limit of about 5ms. Pass configuration through Function input variables or metafields instead of fetching data externally.

The extension must be deployed with shopify app deploy before it appears in Admin → Checkout → Customize. Also verify the correct checkout template is selected and the target in shopify.extension.toml matches the reactExtension target.

No, checkout UI extensions run in a sandboxed Worker without DOM access, so document, window, and localStorage are unavailable. Persist checkout data with useApplyMetafieldsChange and read it back with useMetafield.