CommonJS module output broken after updating from 5.5.4 to 5.6.2 about typescript HOT 7 CLOSED

If you were relying on tsc for dual emit then it might be the easiest to switch to using tshy. If you prefer to handroll the solution then I'd recommend switching to .ts files. I've seen you have post-processing step to fixup extensions and stuff - that step could be reused. It feels that if you just tweak its logic a little bit to handle a different set of files and do a slightly different fixup then it should work. Note that I didn't take too deep into your current setup so what I'm saying is not a guarantee but rather a conclusion drawn solely on a first impression of a very tiny slice of your repo ;p

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bisects to #59479

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So it seems that before this PR it would ignore the fact that your files are authored using .mts and it would allow you to compile that to a commonjs module. With the new version the extension is always taken into consideration, even with "moduleResolution": "node10".

We can trace back this work to #57896 and #58825 . They mention that from now on extensions should be considered in more module modes but they are not as explicit when it comes to what does it mean for moduleResolution modes.

This is a one-line change to fix your issue but the changed code is also almost explicit when it comes to applying the new logic for extensions regardless of moduleResolution... so I think it's best to ping @andrewbranch and defer to his opinion about this.

Minimal repro case:

// @moduleResolution: node10
// @module: commonjs

// @filename: index.mts
export const foo = ''

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Yes, this is working as intended. Compiling .mts files to CommonJS-containing .mjs files was not something we ever intended to support—the fact that it ever worked was a bug / unintentional side effect of other work.

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Related: https://devblogs.microsoft.com/typescript/announcing-typescript-5-6/#respecting-file-extensions-and-package.json-from-within-node_modules

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This issue has been marked as "Working as Intended" and has seen no recent activity. It has been automatically closed for house-keeping purposes.

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Thanks for the answer! I'll review how I dual ship then.

It might be good to document cases where tsc ignore settings. I combed through the documentation first and this behavior didn't appear obvious to me.

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