Upgrading Checkout Blocks app to Polaris web components (2026) - Shopify

What this post added

This post details the upgrade of Checkout Blocks UI extensions to use Preact and Polaris web components with remote-dom, migrating from React and legacy Remote UI bridge. The upgrade involved rewriting code to TypeScript, replacing large libraries like liquidjs with a custom 'droplet' parser and dayjs with an in-house date utility, and aliasing markdown-to-jsx onto Preact. The process was incremental, starting with the smallest extension and gradually migrating others. A key challenge was meeting the 64KB gzip bundle size limit, achieved by dropping react-reconciler and optimizing other dependencies. The post also highlights bugs found and fixed in the platform during this upgrade, including an ID collision issue.