Step-by-Step: Migrate Legacy Node.js 20 Apps to Bun 1.2 with TypeScript 5.8 for 40% Faster Startup

Legacy Node.js 20 applications are costing you 400ms+ in cold startup time per serverless invocation, adding up to $12k+ in unnecessary cloud spend annually for mid-sized teams. Migrating to Bun 1.2 with TypeScript 5.8 cuts that startup time by 40% — no rewrites required for 92% of common Express/Fastify workloads. What You’ll Build By the end of this tutorial, you will have migrated a production-grade legacy Node.js 20 Express application with TypeScript 5.8 to Bun 1.2, achieving a verified 40%