Next.js: `next dev` rewrites tsconfig.json

[borekb]

Bug report

Describe the bug

When I run next dev and some of the "required" Next.js options are missing (even non-critical ones like forceConsistentCasingInFileNames), my tsconfig.json is completely rewritten – my comments are deleted and the file is re-formatted.

To Reproduce

1. Start next dev
2. In tsconfig.json, remove forceConsistentCasingInFileNames and add some comment.
3. Start next dev again
4. Observe that your comment is deleted

Expected behavior

My tsconfig.json is my tsconfig.json 😄.

System information

• OS: macOS
• Version of Next.js: 9.0.2

Additional context

This is from the "TypeScript support is too eager in v9" bucket, IMO. Some more issues I'd regard in that category are e.g. #7687 or #8065 (though they are about different things).

[Janpot]

Also https://github.com/zeit/next.js/issues/8034

[Timer]

If you don't want forceConsistentCasingInFileNames to be true, set it to false instead of deleting the field.

We can look into retaining comments.

[borekb]

It's indeed the same issue as #8034, sorry for not noticing it earlier.

It's OK for Next.js to generate new tsconfig.json when there is none (it's very useful, actually) but overwriting existing files, possibly losing user's work, is something that no tool should do, ever, IMO.

It's also debatable whether Next.js "needs" some of the things it says so, for example, I don't think compilerOptions.isolatedModules is required for Babel and there are serious performance implications of having this turned on, for example, https://github.com/microsoft/TypeScript/issues/32294.

What I'm trying to say: in the end, tsconfig should be user's responsibility. It's fine if Next.js makes reasonable suggestions or does some changes automatically _in some cases_, but it should be careful and conservative, IMO.

[Timer]

@borekb Whether or not compilerOptions.isolatedModules is needed is not debatable -- it is an invariant.

Without this option, TypeScript features that are not supported by Babel will break in unexpected / indeterminate ways and cause countless hours of debugging.

I agree that the config overwrite should never be "destructive", and we'll work on fixing this.

[borekb]

isolatedModules is certainly debatable – for example, we'd like to make a tradeoff between performance and safety in our case. (Next+TS can run perfectly fine without this option.) It's hard for a tool to predict all the possible circumstances and be 100% right all the time.

I just don't think that Next.js should be writing into my source files, especially as a by-product of something as innocent-looking as next dev. I'd prefer any of these:

• Next.js could encode the default tsconfig in some internal way, similarly to how webpack config is hidden from me
• next dev could validate that the configuration is correct and warn me / refuse to start if some critical config is not met
• There could be a manual command like next init-tsconfig

Maybe there are some other options as well. I understand the good intent though and appreciate your effort around this!

[Timer]

I've scheduled this for work in 9.0.7, here's what we'll do:

1. Next.js will not erase comments from your tsconfig.json.
2. Next.js will not override options you have set without prompting you first. You can answer "No" and edit them manually.
3. Next.js will still create a default (but overridable) tsconfig.json for the user if not present.

[majelbstoat]

That's great, thank you 🙏🏼 Can we please also support its (and next-env.dts') location in a configurable place?

[webhacking]

9.0.7-canary.4 works fine.

[jamesreggio]

@Timer I'm a little unclear of your plan from this comment: https://github.com/zeit/next.js/issues/8128#issuecomment-532391600

If I've specified "isolatedModules": false, will it be possible to run next dev without it being overwritten?

[iXmonvi33]

Version 9.1.6 still override everything...

[timneutkens]

So feel free to send a PR, the issue has “good first issue” and “help wanted” labels.

Here’s what you can implement:
https://github.com/zeit/next.js/issues/8128#issuecomment-532391600

[stvmachine]

Yep, 9.1.6 still override.

[timneutkens]

@stvmachine I'm not sure what you're trying to achieve with this comment. You contribute this feature as it's marked as good first issue and help wanted as said before.

[rikkit]

Hey there,

For better or worse, I am a user of ambient const enums. We have a "legacy codebase" and the effort to refactor away from using them has been judged as far too much - we just have too much code that relies on their existence. We have written a Babel plugin to allow Babel to compile code that uses these enums. This is working well with our Storybook project, which is another project that relies on Babel for TypeScript transpilation - from this we know that Babel works just fine with isolatedModules: false.

The remaining hurdle for deploying our new NextJS project is that tsconfig.json must have isolatedModules set to false when running next build. Like @borekb writes above, I appreciate the intention of the NextJS team here, but I would like a way to opt-out of this behaviour without completely turning off TypeScript checking. Babel itself does not forbid this option, so I am not sure that NextJS should enforce it so rigorously.

This issue appears to have been closed automatically as NextJS no longer removes comments, but as of 9.5.3 the isolatedModules option is still mandatory.

Would the team accept a PR that allowed next build to run with isolatedModules: false? I am thinking of a config option ignoreIsolatedModulesError that matches the existing ignoreBuildErrors option which disables type checking completely.