Migrating a Real TypeScript OSS Library from tsup to tsdown

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TypeScript build tool migration guide, actionable for OSS maintainers and platform engineers.

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Why It Matters

For platform engineers maintaining OSS libraries or internal packages, this highlights that build tool migrations require verifying the packed output contract, not just a successful build, to avoid silently breaking consumers relying on specific file paths and module formats.

Key Takeaways

Add a packed-package smoke test to your CI pipeline that compares output filenames and declaration formats against your package.json exports before publishing after any build tool migration.

Summary

Migrating a real TypeScript OSS library from tsup to tsdown revealed that a successful build can silently break the package contract by changing output filenames and declaration formats. The author found that tsdown's default output (index.cjs, index.d.mts, index.d.cts) differed from tsup's (index.js, index.d.ts), requiring explicit config overrides to preserve existing package.json exports for ESM/CJS consumers. A packed-package smoke test was essential to catch these breaking changes before publishing.

Author

nyaomaru