Is it possible to write flat config using TypeScript?

[alexeyr-ci]

From documentation it doesn't seem to be as easy as writing an eslint.config.ts file :) Of course there is an option of compiling TypeScript to get eslint.config.js, but this complicates things too much: e.g. if I commit the compiled file it needs to be kept in sync with TypeScript, if I don't it needs to be regenerated automatically and just running eslint from the command line can fail, etc.

[Rec0iL99]

Hi @alexeyr-ci, currently we don't provide any types for flat config.

[alexeyr-ci]

Yes, that was the question. The way I found is to have eslint.config.js look like this:

require('ts-node').register({
  compilerOptions: {
    module: 'commonjs',
  },
  transpileOnly: true,
});

module.exports = require('./a-typescript-config-file');

[Rec0iL99]

I meant to say that eslint.config.ts is not officially supported by ESLint. Thanks for sharing how to have your config in ts @alexeyr-ci.

[Asaf-S]

I did it differently than @alexeyr-ci, and it also works for me:

1. Add eslint.config.js to .gitignore.
2. Create a file called eslint.config.ts with the following content:

import js from "@eslint/js";
import tsEslint from 'typescript-eslint';

module.exports = tsEslint.config(
  js.configs.recommended,
  ...tsEslint.configs.recommended,
)

3. Add the following packages to package.json by running this command:

npm i -D typescript-eslint @types/eslint__js @typescript-eslint/eslint-plugin

4. Change the lint script in your package.json to run the following command:

tsc --target esnext --module NodeNext --moduleResolution NodeNext eslint.config.ts && eslint . --fix

This will first create the eslint.config.js from the eslint.config.ts file, and then run eslint while using that file.
Meanwhile, while writing the eslint.config.ts file - you'll enjoy type-safety and auto-completion.

[Andre-ADPC]

There is also TypeScript ESLint as an option. Flat-file format config etc. in their words "Monorepo for the tooling that enables ESLint and Prettier to support TypeScript". @Rec0iL99 is it a side-project of ESLint or direct competition to ESLint? I noticed the user base is over 15 mill., so it seems to be popular and well-maintained and part of the OpenCollective initiative.

[alexeyr-ci]

@Andre-ADPC Neither. It's an ESLint plugin which provides rules for TypeScript. It doesn't change how you write the config (except it needed to do some work to support the flat config, like other plugins did).

[voxpelli]

There is an RFC that wants to add native support for writing configs in TS: eslint/rfcs#117

[pcnate]

this would be nice. I'm working with repos that fail status checks when detecting any file with a js extension outside of lib

[karlhorky]

Since ESLint 9.9.0, it's now possible to use the (experimental) eslint.config.ts format:

• https://eslint.org/blog/2024/08/eslint-v9.9.0-released/#experimental-typescript-configuration-files

[omril1]

And now it's without an experimental flag 🎉 🎉 🎉

[yamanidev]

How! @omril1

[karlhorky]

@yamanidev I think @omril1 was referring to ESLint 9.18.0, which made TypeScript configuration files stable:

• https://eslint.org/blog/2025/01/eslint-v9.18.0-released/#stable-typescript-configuration-file-support

Setup instructions can be found in the docs:

• https://eslint.org/docs/latest/use/configure/configuration-files#typescript-configuration-files

[yamanidev]

Thanks for the reply @karlhorky. I am not using it for Node.js or Bun though, I am using it in an Astro SSG application. Do I still need to install the jiti dependency?