You follow data on its round trip: load, check, change, save and print. You read the sample todo file, validate it with a Zod schema so a broken file is refused with each field named, and treat a missing file as no todos yet. You write todos back as indented JSON, add one after the highest id, and mark one done without touching the others. Then you print one line per todo for people, JSON for other programs, and one friendly line on standard error for an unknown id.

You see why one program gets several kinds of checks, each catching what the others miss. You write unit tests with Node's test runner, spawn the real CLI from a test to check its output and exit codes, and measure coverage. You add type tests with Vitest, catch a missing await with Oxlint's type-aware rule that the compiler lets through, and check formatting with Prettier. Then you put all five checks behind npm run check, break the code on purpose to see where it stops, and fix it.

You follow source code on its way to an installed command. You build it with tsdown into plain JavaScript and type declarations, make it runnable with a shebang and a bin entry, and see that only dist, package.json and the README go into the package. You pack a tarball, install it in a clean folder with only Zod alongside, and run todo there. Then you rehearse a publish that uploads nothing, see how versions and a changelog travel with a release, and where to take the tool next.