More issues with Flat Config and TS

[rgant]

Description

Trying to configure eslint-plugin-prefer-arrow-functions in a eslint.config.ts file results in Type issues for meta being possibly undefined.

47:7   error   Type '{ meta: { name?: string | undefined; version?: string | undefined; } | undefined; rules: Record<string, LooseRuleDefinition> | undefined; }' is not assignable to type 'Omit<Plugin, "configs">' with 'exactOptionalPropertyTypes: true'. Consider adding 'undefined' to the types of the target's properties. ​typescript(2375)
                    Types of property 'meta' are incompatible.
                      Type '{ name?: string | undefined; version?: string | undefined; } | undefined' is not assignable to type '{ name?: string | undefined; version?: string | undefined; }'.
                        Type 'undefined' is not assignable to type '{ name?: string | undefined; version?: string | undefined; }'.

This is what the current index.d.ts file says:

import { TSESLint } from '@typescript-eslint/utils';
export declare const meta: {
    name?: string | undefined;
    version?: string | undefined;
} | undefined, rules: Record<string, TSESLint.LooseRuleDefinition> | undefined;

Using eslint version:

$ npx eslint --version
v9.21.0

and latest version of this plugin.

Work Around

import { meta, rules } from 'eslint-plugin-prefer-arrow-functions';

// Should never happen, but tsc doesn't know that.
if (meta == undefined) {
  throw new Error('eslint-plugin-prefer-arrow-functions.meta is undefined');
}

export default tseslint.config(
  {
    plugins: {
      preferArrow: { meta, rules },

[rgant]

Also I'll re-ask for documentation of configuration when using a flat config.

[Kristinita]

Type: Question ❓

@rgant, please say, is the configuration from my patch works for you?

Thanks.

[rgant]

@Kristinita Unfortunately this does not solve the types issues. If you use an eslint.config.ts file instead of mjs then tsc will report the issues above.

However I do really appreciate adding the documentation for flat config!

[Kristinita]

Type: Question ❓

@rgant, do you search on GitHub how another users configured their eslint.config.ts? GitHub contains 2 examples, they may help you.

Thanks.

[rgant]

@Kristinita This is not a problem of configuration. This is a problem of incompatible TypeScript types. The problem is the | undefined for meta (and probably also rules) in index.d.ts. My theory is that if this project provided and tested an eslint flat config (in TypeScript) it would notice and catch these issues.