Upgrading a Legacy Monorepo to React 18: What Nobody Warns You About
React 18 shipped in March 2022. We merged our upgrade in April 2026. If you do that math, yes, we were four years behind. This is a post about what that upgrade actually looked like across a large, multi-generational codebase, and the specific issues that ate most of the time. Spoiler: the “breaking changes” in the official React 18 migration guide are the easy part. The Starting Point Our frontend isn’t one app. It’s four generations of React code living in the same monorepo, piled up over years of product growth: ...