bulk-decaffeinate ( https://github.com/decaffeinate/bulk-decaffeinate https://github.com/decaffeinate/bulk-decaffeinate ) supports an outputFileExtension option

bulk-decaffeinate ( https://github.com/decaffeinate/bulk-decaffeinate https://github.com/decaffeinate/bulk-decaffeinate ) supports an outputFileExtension option that can make it so the resulting files end in .ts. But really, the main thing to do when converting JavaScript to TypeScript is adding types, which decaffeinate is much less qualified to do. But if you're going to move from CoffeeScript to TypeScript, JavaScript is probably a pretty important intermediate step.