refactor: update TypeScript to v6

[lumirlumir]

Prerequisites checklist

• I have read the contributing guidelines.

AI acknowledgment

• I did not use AI to generate this PR.
• (If the above is not checked) I have reviewed the AI-generated content before submitting.

What is the purpose of this pull request?

This PR updates TypeScript to v6.

The following two changes mainly affected us during the upgrade to TypeScript v6.

• strict mode is now the default.

https://devblogs.microsoft.com/typescript/announcing-typescript-6-0/#simple-default-changes

• rootdir now defaults to .

https://devblogs.microsoft.com/typescript/announcing-typescript-6-0/#rootdir-now-defaults-to-.

What changes did you make? (Give an overview)

I've tried to retain strict mode behavior as much as possible and only disabled it where it produced too many errors.

src/index.js

The logic below produced a type error after upgrading to TypeScript v6, so I used an object getter instead to avoid the error and to avoid the block-syntax workaround.

json/src/index.js

Lines 39 to 42 in c9e816f

	// eslint-disable-next-line no-lone-blocks -- The block syntax { ... } ensures that TypeScript does not get confused about the type of `plugin`.
	{
	plugin.configs.recommended.plugins.json = plugin;
	}

Freezing the configuration object wasn't considered a breaking change in eslint/eslint#20116, so using a getter here seems fine, but I'd welcome other opinions.

I've also verified that the output index.d.ts file is the same as before.

tests/types/tsconfig.json

• Re-enabled strictNullChecks and useUnknownInCatchVariables after they were turned off in root level tsconfig.json.

package.json

• Added @types/natural-compare as a devDependency to fix a type error introduced by the TypeScript v6 breaking change.

tsconfig.json

• Added "rootDir": "./src" to address the breaking change in TypeScript v6.
• Turned off strictNullChecks because adopting it would require extensive changes and it was previously disabled.
• Turned off useUnknownInCatchVariables because adopting it requires some changes and it was previously disabled.

Related Issues

N/A

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

N/A

[Copilot]

Pull request overview

Updates the repository’s TypeScript toolchain to v6 and adjusts configuration/type-checking to accommodate TypeScript v6 default behavior changes (notably strict and rootDir defaults), while keeping emitted typings and runtime behavior stable.

Changes:

• Bumps typescript to ^6.0.2 and adds @types/natural-compare for stricter type-checking.
• Updates root tsconfig.json to pin rootDir and selectively disable specific strictness flags.
• Adjusts src/index.js recommended config to reference the plugin via a getter (avoiding TS v6 inference issues) and tightens tests/types/tsconfig.json strictness.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated no comments.

File	Description
tsconfig.json	Pins rootDir to keep dist/ layout stable under TS v6 and overrides specific strict options.
tests/types/tsconfig.json	Re-enables strictNullChecks and useUnknownInCatchVariables for type tests despite root-level overrides.
src/index.js	Replaces post-definition assignment with a getter-based plugins.json reference to avoid TS v6 typing issues.
package.json	Updates TypeScript to v6 and adds missing ambient types for natural-compare.

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[snitin315]

LGTM, thanks!

[nzakas]

Practical question: TypeScript v6 just came out, do we need to rush to support it? What happens if someone with TypeScript v6 tries to use a v5 package? Is there any end-user benefit or is this just about upgrading to the latest?

[lumirlumir]

Practical question: TypeScript v6 just came out, do we need to rush to support it? What happens if someone with TypeScript v6 tries to use a v5 package? Is there any end-user benefit or is this just about upgrading to the latest?

I first tried upgrading TypeScript to v6 to see whether the deprecation of some options in v6 would affect our use case and end users, since those deprecations might have an impact.

After investigating the codebase and reviewing the TypeScript v6 announcement, my understanding is that our current tsconfig.json setup is not affected by the changes in TypeScript v6. I verified that the generated .d.ts files did not change after upgrading to TypeScript v6 and the .js files only include refactoring and contain no logic changes. (But, I only verified this against the Markdown, JSON, and CSS language plugins, so other repositories may need to be checked.)

In summary, I think updating to TypeScript v6 will not affect end users because the output files remain unchanged. So, as a result, this PR simply upgrades to the latest version.