checkJs: allow file extensions other than .js

First of all, this option is super exciting! Thank you so much for the TypeScript team to make such an incredible tool ❤️

Surprisingly enough, this week I wanted to have type check in a *.jsfl file (Scripting for Adobe Animate) and it worked pretty well using Facebook’s Flow. They have an option to configure a different file format other than .js.

It would be nice to allow this sort of configuration for TypeScript as well. I’ve naively tried to use "include":["./src/*.jsfl"] but it didn’t worked. 😅

Or is there a hidden option for this? Thanks!

[SMotaal]

I did a little experiment a while back where I carefully modified TypeScript’s source to add support for .mjs. I did this in a few hours and that included getting all the tests running.

I honestly had assumed from the push back that it is not that simple, but then I found that it is.

It became clear from further interactions that while it can be done, it cannot be supported in a way that meets the quality expectations from TypeScript.

If my PR would have been accepted, could my implementation have broken existing projects? In my opinion, that is the only problem that I completely considered beyond my scope as an external contributor.

Once again, imho, as a user not interested in or not expecting this new behaviour, all I expect from TypeScript is to lock it behind a flag so that only those who opt in and understand and accept the ramifications of “experimental support for other extensions” are affected by it. The other guy still wants it and the fact that it is not being addressed by TypeScript on par with the implications of the issue are extremely disappointing.

https://github.com/Microsoft/TypeScript/pull/23678

[omeid]

The checkJs and allowJs merely adds complexity to the compiler, the simple solution would be to allow users to configure what files need be treat as TypeScript.