I think because a reader who doesn't want to use TypeScript can trivially remove the type annotations, so they're not actually a barrier to using this code in J

I think because a reader who doesn't want to use TypeScript can trivially remove the type annotations, so they're not actually a barrier to using this code in JavaScript. Some people might also be suspicious that the reason to bring this up is to imply hostility to JavaScript tooling that requires a build step, since it's become popular to hate on this.