Jest with TypeScript do not report any type errors

[gyzerok]

Hello, here is my bug report.

Currently when you use TypeScript in a way shown in example you will not be reported if there are type errors in tests.

Reproduce:

git clone git@github.com:gyzerok/jest-ts-bug.git
cd jest-ts-bug
npm install
npm test

To verify that there is actual type error:

npm run typecheck

I expect type errors to be reported while running tests. Preferably well-formatted.

[cpojer]

How are type errors thrown by the TypeScript compiler? If it throws a type error on a file, Jest should catch that and show it inline. If the type error is thrown in a file that hasn't changed, Jest will not compile that particular file because it has a transform cache based on when files change. If that's the case, I'm afraid there is nothing we can do except for you to always run with the (slow) --no-cache option.

[gyzerok]

Hello @cpojer

One should gather type errors by himself inside preprocess script. Currently the function which is used in example do not collect any diagnostics. And even if it would there would be no type errors within it. Note transpile and transpileModule are basically the same function with different signatures.

Also in case it throws there is no source maps, so errors shows completely random line numbers. At least for source maps there is solution.

By writing this I am now thinking that it has nothing to do with Jest. But still maybe it would be nice to use ts-jest in the example instead of providing preprocessor which is kind of broken in so many ways. And of course it would be nice to be explicitly notified about problems with type errors in TypeScript, so one could see the trade-offs before putting his time in technology adoption.

[cpojer]

Feel free to send a PR to change the TypeScript example to use ts-jest. Indeed, TypeScript support is not "official". You can use any transformer and TS is just one of them – the problems of showing type errors or source maps etc. are entirely on the transformation side and Jest merely provides hooks for these things.

[gyzerok]

@cpojer actually if Jest would provide ability to use stuff like babel-register or ts-node/register, like mocha do, issue would be solved I guess.

[cpojer]

That would be incredibly slow as we'd have to recompile everything on every run. Jest caches transform results.

[pelotom]

BTW, ts-jest doesn't currently display type errors either. I've opened this ticket to request that feature.

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