Will the "eslintConfig" key in package.json files still work in ESLint v9?

[dustin-ruetz]

Context

As of this writing using ESLint v8.56.0 I use the following configuration in my package.json files:

{
  "scripts": {
    "lint": "eslint --config ./package.json",
    "lint:check": "npm run lint -- ./",
    "lint:fix": "npm run lint -- --fix ./"
  },
  "eslintConfig": {
    "extends": "./node_modules/dev-config/lib/eslint.config.js"
  }
}

Using the eslintConfig.extends key to point to the custom path of the eslint.config.js file is very helpful because it allows me to define the path to the configuration file once, and both the eslint CLI commands and the ESLint VS Code extension work as expected. As you can see by the path, this eslint.config.js file is published as part of a development config package that all of my other projects use.

Documentation Contradictions

The reason I'm starting this discussion thread is because while the v8 /docs/latest/use/configure/configuration-files page lists the package.json file as an option, the v9 /docs/next/use/configure/configuration-files page only mentions the eslint.config.js file and that it must be located in the root directory. With that being said, the v9 /docs/next/use/command-line-interface#-c---config page makes it sound like package.json configuration will still be supported:

-c, --config example

npx eslint -c ~/my-eslint.json file.js

This example uses the configuration file at ~/my-eslint.json.

If .eslintrc.* and/or package.json files are also used for configuration (i.e., --no-eslintrc was not specified), the configurations are merged. Options from this configuration file have precedence over the options from .eslintrc.* and package.json files.

Questions

So with all of this context presented, my question is:

1. Will the "eslintConfig" key in package.json files still work in ESLint v9?

And as a follow-on question:

2. If "yes" to the above, will the data structure change to match the flag config changes? I'm assuming that this would have to be a "yes" considering that v8's extends key will no longer be a config option as of v9, so perhaps the v9 implementation would look something like this?

{
  "eslintConfig": ["./node_modules/dev-config/lib/eslint.config.js"]
}

[fasttime]

Configuration settings in package.json are not supported in the new config system (the default for v9). I guess the migration guide could be more explicit about that.

[dustin-ruetz]

Hi @fasttime, thanks for replying 🙏

Would you happen to have any insight on if the ESLint team would consider supporting the "eslintConfig" key in package.json files in v9?

To clarify:

1. I understand that it couldn't function the same way as it did in v8 due to A) the various API changes, and B) the fact that v9's flat config files now rely on a lot of JavaScript language features that can't be used in static JSON files (i.e. being able to import plugins, using the object spread syntax to extend recommended configuration settings, etc.).
2. As suggested in my original post, being able to set "eslintConfig" (either as a single string or an array of strings pointing to the path(s) of non-root configuration file(s)) would make intuitive sense; while we may not be able to write inline JSON configuration any more, it feels like we should be able to pass a path to custom location(s) of the flat config file(s).
3. I wouldn't consider a lack of support for the "eslintConfig" key as blocking the stable release of v9.

I can think of the following advantages for continuing to support a simplified (i.e. path only) implementation of this functionality:

1. Being able to specify a custom location of the configuration file is supported when running eslint as a CLI tool using the --config flag, and it feels like this is a valuable use case for the ESLint extension to support as well.
2. Supporting config files that are not located in the root of the repo can help keep repo roots cleaner, i.e. less config file clutter.
3. I've seen a lot of repos using the "eslintConfig" key in their package.json files.
4. A number of other popular tools (notably Jest, lint-staged and Prettier) continue to support package.json configuration.

[fasttime]

The main reason for removing the "eslintConfig" key lookup was to avoid the complexity of the legacy config system, where many ways existed to do the same thing. This is one of the aims stated in this blog post and is mentioned in the original RFC.

Some of the initial decisions in the RFC have been revisited later because they turned out impractical or were posing challenges to the ecosystem, but it looks like this does not apply in your case. So I would not expect the team to consider reinstating support for the "eslintConfig" key in package.json files for the advantages you mention.

I think the best you could do is adding a eslint.config.js file to the root of your repo where you re-export the common config, i.e.

// ./eslint.config.js
module.exports = require("dev-config/lib/eslint.config.js");

This should work also in vscode-eslint when they add support for ESLint v9.

[dustin-ruetz]

Thanks for providing this additional context @fasttime, I really appreciate it 🙏

I definitely understand the desire to simplify and streamline the implementation, and supporting a lot of configuration methods adds much more complexity.

---

All of my new codebases are now fully ESM, but I've tested the following with ESLint v9 and have confirmed that both the CLI tool and extension work with these changes:

// eslint.config.js
export {default} from "dev-config/lib/eslint.config.js";

// package.json
"scripts": {
  "lint:check": "eslint ./",
  "lint:fix": "eslint --fix ./"
},

---

If anyone is reading this discussion in the future, I did figure out another way to have the VS Code extension work with a non-root configuration file located in an arbitrary path. (This solution is specific to VS Code, but I'm sure other editors offer similar settings.) I'm not sure if this method will continue working when ESLint v9 reaches stable release status and/or if there will be further API changes being made to the extension, but I assume that @dbaeumer would know the most about this.

// .vscode/settings.json
{
  "eslint.options": {
    "overrideConfigFile": "./node_modules/dev-config/lib/eslint.config.js"
  }
}