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

This post details the migration of Checkout Blocks UI extensions from React/JSX and legacy Remote UI to Preact/TypeScript and remote-dom with Polaris web components. It highlights the reduction in bundle size (40-85%) and load times (ELT P50 -8%, P90 -7%) achieved by switching to Preact, adopting TypeScript, and replacing large libraries like liquidjs with a custom parser ('droplet') and dayjs with an in-house utility. The post also discusses the incremental upgrade strategy, the challenges of meeting the 64KB gzip bundle size limit, and the discovery and upstreaming of platform bugs.