fix: add support for exactOptionalPropertyTypes in type definitions

[sebastian-altamirano]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

When using @eslint/markdown as a plugin in an ESLint config file, TypeScript throws the following error if the exactOptionalPropertyTypes compiler option is enabled:

Type 'typeof plugin' is not assignable to type 'Plugin'.
...
    Type 'undefined' is not assignable to type '(string | string[])[]'.ts(2322)

For example, this can be triggered by using plugins: { markdown } in:

import markdown from "@eslint/markdown";
import { defineConfig } from "eslint/config";

export default defineConfig([
    {
        extends: ["markdown/recommended"],
        files: ["**/*.md"],
        language: "markdown/gfm",
        plugins: { markdown },
        rules: {
            "markdown/no-duplicate-headings": "error",
            "markdown/no-html": "error",
        },
    },
]);

This PR fixes the issue.

What changes did you make? (Give an overview)

I noticed that the Plugin type was unused, so I updated it to be compatible with exactOptionalPropertyTypes and began using it, without breaking existing tests.

Related Issues

-

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

-

[linux-foundation-easycla]

• ✅login: sebastian-altamirano / (9674f9f)

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[lumirlumir]

Hello @sebastian-altamirano, thanks for opening a PR.

Could you provide a reproducible link and your TypeScript tsconfig.json settings? I tried to reproduce the issue, but couldn’t reproduce it on my end.

[sebastian-altamirano]

Sure, I can't create a reproducible link right now, I'll do it later if you still need it, but I can pass you the tsconfig.json I used to reproduce the problem:

{
	"extends": "@tsconfig/strictest/tsconfig.json",
	"compileOnSave": false,
	"compilerOptions": {
		"baseUrl": "./",
		"outDir": "./dist/out-tsc",
		"strict": true,
		"skipLibCheck": true,
		"isolatedModules": true,
		"esModuleInterop": true,
		"experimentalDecorators": true,
		"moduleResolution": "bundler",
		"importHelpers": true,
		"target": "ES2022",
		"module": "ES2022",
		"paths": {
			"@core/*": ["src/app/core/*"],
			"@layout/*": ["src/app/layout/*"],
			"@page/*": ["src/app/page/*"],
			"@shared/*": ["src/app/shared/*"],
			"@types/*": ["src/app/types/*"]
		}
	},
	"angularCompilerOptions": {
		"enableI18nLegacyMessageIdFormat": false,
		"strictInjectionParameters": true,
		"strictInputAccessModifiers": true,
		"strictStandalone": true,
		"strictTemplates": true
	}
}

You can test it with the following eslint.config.mjs:

import markdown from "@eslint/markdown";
import { defineConfig } from "eslint/config";

export default defineConfig([
    {
        extends: ["markdown/recommended"],
        files: ["**/*.md"],
        language: "markdown/gfm",
        plugins: { markdown },
        rules: {
            "markdown/no-duplicate-headings": "error",
            "markdown/no-html": "error",
        },
    },
]);

Then you should see this error:

If you add "exactOptionalPropertyTypes": false to the compilerOptions of the tsconfig.json the error disappears.

[sebastian-altamirano]

Here's a repro https://stackblitz.com/edit/vitejs-vite-gantpukn?file=eslint.config.mjs,tsconfig.json&view=editor

I'm not sure why the StackBlitz IDE doesn't display the error, but if you run npx tsc you should be able to see it. Then if you can change exactOptionalPropertyTypes to false in tsconfig.json and rerun the command, the error will no longer be reported.

[nzakas]

@sebastian-altamirano please open an issue on https://github.com/eslint/eslint/issues. The Plugin type exported from eslint is the thing that needs updating to address this.

[sebastian-altamirano]

I created the issue on #402.

[sebastian-altamirano]

After some discussion, I believe the issue should be addressed on the plugin side:

• Bug: exactOptionalPropertyTypes causes type errors when using plugins that were not built with this option #402: @eslint/markdown does not provide a type definition for plugin, so TypeScript infers one. The inferred type works correctly unless exactOptionalPropertyTypes is enabled. Other ESLint plugins that rely on TSESlint type definitions encounter the same issue because those types are incompatible with native ESLint types.
• fix: add explicit undefined to Linter.Config optional properties eslint#19775: The current ESLint types are correct.

@nzakas, given this context, do you think this PR can be reopened?