When you store JSON locally - app data, config files, browser storage - the schema inevitably changes between versions.

New fields get added, old ones get renamed, formats evolve. Without a migration system, you end up with brittle ad-hoc upgrade code or broken user data.

I built json-up, a small TypeScript library that lets you define a chain of versioned migrations that transform JSON data step by step. It's type-safe, so the compiler catches mismatches between versions.

Originally, I needed this for upgrading local app data between mobile app releases for Whisper, but it works for anything where JSON schemas drift over time. Happy to hear what you think.

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json-up: https://github.com/Nano-Collective/json-up Whisper: https://usewhisper.org