TypeScript 2.3 checkJs support

[OliverJAsh]

I am trying to use the new checkJs flag in TypeScript 2.3. When I run tsc standalone, I get errors from my JS files, however when I run ts-loader, I get no errors from my JS files.

Any idea what might be causing this?

[johnnyreilly]

Are you using TypeScript 2.3 in your package.json?

[OliverJAsh]

I am. I checked with yarn list typescript and only 2.3 comes up, so there's definitely not multiple versions of TS.

[OliverJAsh]

I created a small test case: OliverJAsh/webpack-ts-check-js@c2a5d4e

In this example, the entry file is JS, which depends on a TS file. Transpilation works, but I'm expecting to see typecheck errors (checkJs) and there are none.

I also tried changing the entry file to TS, depending on a JS file. However that also failed to show any typecheck errors.

[schmuli]

After looking through the source code, and debugging the loader as well, I think I have identified what can be changed to better support JS errors in checkJs mode:

https://github.com/TypeStrong/ts-loader/blob/master/src/after-compile.ts#L129:

Instead of filtering for only [.d.ts, .ts, .tsx], use instance.compilerOptions.checkJs to determine the correct filter to use, similar to how scriptRegex is calculated in https://github.com/TypeStrong/ts-loader/blob/master/src/instances.ts#L97.

[johnnyreilly]

@schmuli - sounds promising! We welcome PRs 👍 😄