Originally published on Hashnode . Cross-posted for the DEV.to community. The first time I tried to migrate a large JavaScript codebase to TypeScript, I made the classic mistake. I planned a six-week migration project, kicked off with a team meeting, started at the top of the directory tree, and got about 4% through before the project stalled. Other work kept coming in. The migration sat in a long-running branch that diverged from main every day. Three months later, I gave up and merged the partial work back with a lot of any types and a lot of regrets. The second time I tried, I had learned the lesson. Big-bang migrations do not work in production codebases that have to keep shipping. The migration has to be incremental, and it has to be done in a way that lets the rest of the team keep working without coordination overhead. The tools that exist for this kind of migration are good but require enormous amounts of human time to apply correctly across a large codebase.…