Added ts-jest

[emin93]

In this PR I re-added ts-jest to enable out of the box jest testing with TypeScript.

[JReinhold]

Just to get a deeper understanding, may I ask why this change is necessary? 🙂

[emin93]

I found out that ts-jest can't recognize JSX/TSX when this is set to react-native. You can easily reproduce it by switching it back to react-native and running npm test / yarn test, it will throw a unexpected Token <-error. I really appreciate your interest 😄

[JReinhold]

Oh great, I've had that exact problem before, I din't know it had anything to do with that setting. I'll have to try it out tomorrow!

[JReinhold]

This works great in my current project. Although I had to add an extra configuration to make it work with some of the more advanced libraries like react-navigation and react-gesture-handler. I'll leave the extra configuration here, in case anyone else comes by with the same problem:

Jest config (in package.json if using this template), gotten from kulshekhar/ts-jest#604 (comment):

...
"transformIgnorePatterns": ["/node_modules/(?!react-native)/.+"],

[emin93]

Oh that's very good to know! If I understood the regex correctly, it tells Jest to ignore everything that starts with react-native in the node_modules directory? Wouldn't it be better to ignore everything inside the node_modules directory?

[JReinhold]

It's actually the exact opposite, it tells jest to ignore all modules EXCEPT react-native-files. Notice the ! at the beginning. I think jest defaults to ignore all node_modules, so this regex is basically whitelisting react-native in the transformation.

[emin93]

Sorry for my delayed answer, had quite a lot of action going on 😅

Ah I see, thanks for the explanation! I'm curious to know why ts-jest needs those files and if it makes sense to include that whitelisting config as a default in the template?

[JReinhold]

Yeah, I can't really tell for sure, why it needs that. I see that line in a lot of places and example repos, but can't find a specific answer to WHY its there.

IMHO it should be part of the default, because my experience is that you won't get very far without it. As you've experienced, you can test the classic hello world app without the transformIgnorePatterns, but anything beyond that will break in my experience.