feat: Add support for TS config files

[aryaemami59]

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What changes did you make? (Give an overview)

This PR:

• Adds support for TypeScript config files (eslint.config.ts, eslint.config.mts, eslint.config.cts)
• Adds jiti as a dependency.
• Conditionally imports jiti based on whether the config file's extension ends with ts.

I went through a lot of different possible ways of implementing it, and this by far seems to be the simplest way of doing it without changing too much of the internals of the package or adding too many dependencies.

Is there anything you'd like reviewers to focus on?

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[cany748]

Wouldn't it be better to use Jiti for this? I think it's perfect for this, it's used in Nuxt and TailwindCSS to load configuration from Typescript files. It is much smaller and faster than Typescript.

[aladdin-add]

the first-class ts config support has been discussed in #12078 eslint/rfcs#50, and seems we didn't come to an agreement to accept it.

[aladdin-add]

it should be possible to use ts config in the current config. The easiest way I can think of is to use --config and --loader, something like:

NODE_OPTIONS=--loader=tsx eslint --config=eslint.config.ts xxx

[aryaemami59]

@cany748 I had to research it to make sure it doesn't create import side effects. But it seems like you were right, it does what I wanted tsx to do without creating side effects.

[aryaemami59]

@aladdin-add

it should be possible to use ts config in the current config. The easiest way I can think of is to use --config and --loader, something like:

NODE_OPTIONS=--loader=tsx eslint --config=eslint.config.ts xxx

That doesn't work you'll get:

Error: tsx must be loaded with --import instead of --loader

And if you use

NODE_OPTIONS=--import=tsx eslint --config=eslint.config.ts

you'll get a YAMLException error if you have a type annotation anywhere in the config file. And if you don't have a type annotation anywhere you'll still get this error:

Error: ESLint configuration in --config is invalid:
        - Property "" is the wrong type

[aladdin-add]

I mean flat config (which is the default in eslint v9). If you are using eslint v8, the default is eslintrc, and you need to specify ESLINT_USE_FLAT_CONFIG=true

[aryaemami59]

@aladdin-add

I mean flat config (which is the default in eslint v9). If you are using eslint v8, the default is eslintrc, and you need to specify ESLINT_USE_FLAT_CONFIG=true

That still won't work with .mts or .cts extensions, it also doesn't respect the type field in package.json.

Out of curiosity what is your opinion in the current solution this PR provides?

[aladdin-add]

I think it needs a rfc to evaluate: https://github.com/eslint/rfcs

My hope:

• it's opt-in, not default
• 0-overhead for js users
• no bind tsx/ts-node/..., worth exploring a standard way to let users make the choice which one to use.

[nzakas]

I agree with @aladdin-add -- I think we're now at the point where it makes sense to put together an RFC discussing the design with the potential alternatives.

[aryaemami59]

@nzakas

I agree with @aladdin-add -- I think we're now at the point where it makes sense to put together an RFC discussing the design with the potential alternatives.

Fair enough, I will put up an RFC.

[sam3k]

Per 2024-03-07 TSC Meeting, we will remove from v9 board for now and wait for an RFC from @aryaemami59

[aryaemami59]

RFC has been submitted: eslint/rfcs#117