feat: Allow function-based customSyntax options #357

Copilot · 2026-01-07T16:44:42Z

Changes

Summary

Removed redundant function-to-object conversion logic in parse() method as normalizeLanguageOptions() already handles this
Fixed linting errors by converting arrow functions to properly documented function declarations
Updated test to normalize language options before parsing when using function-based customSyntax
Added TypeScript type cast to indicate customSyntax is always an object after normalization
Ran prettier to format files

Addressed Feedback

Code review comment: Removed lines 232-236 in parse() method - the conversion is now only done in normalizeLanguageOptions()
Linting errors: Fixed func-style and JSDoc errors by converting arrow functions to function declarations with proper documentation
Formatting: Ran prettier to format all files

All tests pass (963 passing) and linting passes with no errors.

Original prompt

This section details on the original issue you should resolve

<issue_title>Change Request: Allow function customSyntax options</issue_title>
<issue_description>### Environment

HEAD

What problem do you want to solve?

The CSS plugin currently only accepts objects as languageOptions.customSyntax. This is primarily so we can serialize languageOptions for various reasons.

However, CSSTree also allows function-based customSyntax, which is helpful when you want to extend the base syntax in some way. This also ensures that the package providing the custom syntax doesn't need a direct dependency on CSSTree to work.

I'd like to extend languageOptions.customSyntax to accept functions.

What do you think is the correct solution?

Update CSSLanguage so that languageOptions.customSyntax can be a function. This should match the SyntaxExtensionCallback type.

In order to make it serializable, instead of passing directly into CSSTree, we'll instead import the default syntax:
import defaultSyntax from "@eslint/css-tree/definition-syntax-data";
Then call the custom syntax function and pass in defaultSyntax, then evaluate the resulting object before passing it to CSSTree. This mimics what CSSTree does internally with a function syntax extension

Participation

I am willing to submit a pull request for this change.

Additional comments

No response</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes Change Request: Allow function customSyntax options #356

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eslint-github-bot · 2026-01-07T16:44:47Z

Hi @Copilot!, thanks for the Pull Request

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eslint-github-bot · 2026-01-07T16:50:12Z

Hi @copilot!, thanks for the Pull Request

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Co-authored-by: nzakas <[email protected]>

eslint-github-bot · 2026-01-07T16:59:11Z

Hi @Copilot!, thanks for the Pull Request

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eslint-github-bot · 2026-01-07T16:59:14Z

Hi @Copilot!, thanks for the Pull Request

The pull request title isn't properly formatted. We ask that you update the pull request title to match this format, as we use it to generate changelogs and automate releases.

The commit message tag wasn't recognized. Did you mean "docs", "fix", or "feat"?
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Co-authored-by: nzakas <[email protected]>

eslint-github-bot · 2026-01-07T17:02:03Z

Hi @copilot!, thanks for the Pull Request

The pull request title isn't properly formatted. We ask that you update the pull request title to match this format, as we use it to generate changelogs and automate releases.

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eslint-github-bot · 2026-01-07T17:02:06Z

Hi @Copilot!, thanks for the Pull Request

The pull request title isn't properly formatted. We ask that you update the pull request title to match this format, as we use it to generate changelogs and automate releases.

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eslint-github-bot · 2026-01-07T17:08:08Z

Hi @Copilot!, thanks for the Pull Request

The pull request title isn't properly formatted. We ask that you update the pull request title to match this format, as we use it to generate changelogs and automate releases.

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Co-authored-by: nzakas <[email protected]>

eslint-github-bot · 2026-01-07T17:10:09Z

Hi @copilot!, thanks for the Pull Request

The pull request title isn't properly formatted. We ask that you update the pull request title to match this format, as we use it to generate changelogs and automate releases.

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eslint-github-bot · 2026-01-07T17:10:12Z

Hi @copilot!, thanks for the Pull Request

The pull request title isn't properly formatted. We ask that you update the pull request title to match this format, as we use it to generate changelogs and automate releases.

The commit message tag wasn't recognized. Did you mean "docs", "fix", or "feat"?
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eslint-github-bot · 2026-01-07T17:12:15Z

Hi @copilot!, thanks for the Pull Request

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nzakas

@copilot fix the linting errors.

nzakas · 2026-01-07T21:25:26Z

src/languages/css-language.js

+		// Convert function-based customSyntax to object if needed
+		let customSyntax = languageOptions.customSyntax;
+		if (typeof customSyntax === "function") {
+			customSyntax = customSyntax(defaultSyntax);
+		}


@copilot we don't need these. normalizeLanguageOptions() ensures languageOptions.customSyntax is an object.

Removed the redundant conversion logic in parse(). The function-to-object conversion now only happens in normalizeLanguageOptions(). Fixed in commit 5dcc991.

Co-authored-by: nzakas <[email protected]>

Copilot · 2026-01-07T21:39:20Z

@copilot fix the linting errors.

Fixed linting errors by converting arrow functions to function declarations with proper JSDoc documentation. All linting errors are now resolved. Fixed in commit 5dcc991.

nzakas

LGTM.

nzakas · 2026-01-07T22:56:34Z

@copilot run prettier to properly format the files.

Co-authored-by: nzakas <[email protected]>

Copilot · 2026-01-07T22:59:27Z

@copilot run prettier to properly format the files.

Ran prettier to format all files. Fixed in commit 11746fe.

Initial plan

03a6f96

Copilot AI assigned Copilot and nzakas Jan 7, 2026

github-project-automation bot added this to Triage Jan 7, 2026

github-project-automation bot moved this to Needs Triage in Triage Jan 7, 2026

Copilot started work on behalf of nzakas January 7, 2026 16:45 View session

Allow function customSyntax options

c4c0b08

Co-authored-by: nzakas <[email protected]>

Address code review feedback

e21909e

Co-authored-by: nzakas <[email protected]>

Add documentation for function-based customSyntax

c34f975

Co-authored-by: nzakas <[email protected]>

Copilot AI changed the title ~~[WIP] Update customSyntax to accept function options~~ Allow function-based customSyntax options Jan 7, 2026

Copilot AI requested a review from nzakas January 7, 2026 17:11

Copilot finished work on behalf of nzakas January 7, 2026 17:12

nzakas changed the title ~~Allow function-based customSyntax options~~ feat: Allow function-based customSyntax options Jan 7, 2026

eslint-github-bot bot added the feature label Jan 7, 2026

nzakas marked this pull request as ready for review January 7, 2026 21:25

nzakas reviewed Jan 7, 2026

View reviewed changes

nzakas moved this from Needs Triage to Implementing in Triage Jan 7, 2026

Copilot started work on behalf of nzakas January 7, 2026 21:28 View session

Remove redundant conversion logic and fix linting errors

5dcc991

Co-authored-by: nzakas <[email protected]>

Copilot AI requested a review from nzakas January 7, 2026 21:40

Copilot finished work on behalf of nzakas January 7, 2026 21:40

nzakas approved these changes Jan 7, 2026

View reviewed changes

nzakas moved this from Implementing to Merge Candidates in Triage Jan 7, 2026

Copilot started work on behalf of nzakas January 7, 2026 22:57 View session

Run prettier to format files

11746fe

Co-authored-by: nzakas <[email protected]>

Copilot AI requested a review from nzakas January 7, 2026 23:00

Copilot finished work on behalf of nzakas January 7, 2026 23:00

nzakas approved these changes Jan 8, 2026

View reviewed changes

nzakas merged commit 3479200 into main Jan 8, 2026
22 checks passed

nzakas deleted the copilot/allow-function-customsyntax-options branch January 8, 2026 21:01

github-project-automation bot moved this from Merge Candidates to Complete in Triage Jan 8, 2026

eslintbot mentioned this pull request Jan 8, 2026

chore: release 1.0.0 🚀 #322

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feat: Allow function-based customSyntax options #357

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Changes

Summary

Addressed Feedback

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What do you think is the correct solution?

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