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Migrating a Real TypeScript OSS Library from tsup to tsdown
The article narrates the author's experience migrating a TypeScript open-source library from tsup to tsdown. Initially thought to be straightforward, the migration revealed challenges in maintaining the package's contract with consumers. The author underscores the importance of testing and verifying the output to ensure compatibility with existing users, emphasizing that merely achieving a successful build is insufficient.
MonitorConfidence: MediumMid term horizonStory: First report
EvidenceOn record sourcesIndependent evidenceNo quotes
Published 7 Oct 2026, 16:51 UTC8 min read
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Brief
NewsBite reading:Migrating TypeScript Library from tsup to tsdown: Key Lessons Learned
TL;DR
- Migration from tsup to tsdown posed challenges in maintaining package contracts.
- Important testing strategies included smoke tests to verify consumer experience.
- Build configuration changes were small but critical for compatibility.
- Output size and performance implications were considered during migration.
- The author encourages others using tsup to transition to the faster tsdown.
Interesting facts
- The migration revealed silent changes that could break existing contracts.
- Testing the output as an artifact was essential to ensuring user compatibility.
- While the new bundler was faster, its output size was larger in raw JavaScript.