We still typecheck and build in different steps due to speed; we can build our JS much faster than TS can with Babel + Webpack. We also have a bunch of old custom features that our build system needs to support, so bolting them onto tsc was a non-starter.

Plus, we have to support some fairly old browsers (we only recently dropped support for IE11), so Babel is certainly doing a lot of transforms on the bundles we ship to older browsers.

Separately, do you mean Terser instead of ESLint? ESLint isn’t really part of our build process.