I wish there was a better testing story for TypeScript. ts-jest is slow because it does not benefit from incremental compilation. Having tsc compile everything creates a dist/ folder with tests, fixtures, etc and causes all sorts of subtle issues to be worked around. Also, trying to use tsc-watch and something like jests watch runs into all sorts of weird race conditions though at least the build and the tests complete in about 1 second.

I looked into combining tsc-watch and jest, but jest does not currently support a public API. tsc-watch already emits an event on success which should be able to trigger (an already running!) test runner to incrementally test again.