fix: parse filter elements(...) syntax in f-children directive

[Prasad48432]

Pull Request

📖 Description

This PR fixes parsing of the filter elements(...) syntax for the declarative f-children directive.

Previously, TemplateParser passed the entire directive value directly to children(), causing expressions such as f-children="{childItems filter elements(li)}" to be treated as a literal property name instead of parsing the filter. As a result, no element filter was applied.

This change extracts the filter parsing into a shared helper used by both the children and slotted directive cases. When a filter elements(...) clause is present, the parser now constructs the appropriate options object and passes it to children(), while preserving the existing behavior for directives without filters.

🎫 Issues

Closes #7631

👩‍💻 Reviewer Notes

• The change is isolated to packages/fast-element/src/declarative/template-parser.ts.
• Parsing of the filter elements(...) syntax is now shared between the children and slotted directive implementations to avoid duplicate logic.
• Existing behavior for f-children="{property}" is preserved. The options object is only used when a filter is specified.

📑 Test Plan

Added declarative parser tests covering:

• f-children="{listItems}" resolves to a ChildrenDirective with the expected property and no filter.
• f-children="{childItems filter elements()}" produces a filter that accepts element nodes and rejects text nodes.
• f-children="{childItems filter elements(li)}" produces a selector-based filter that accepts <li> elements and rejects other elements and text nodes.

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the [CONTRIBUTING](https://github.com/microsoft/fast/blob/main/CONTRIBUTING.md) documentation and followed the [standards](https://github.com/microsoft/fast/blob/main/CODE_OF_CONDUCT.md#our-standards) for this project.

Agents

• I have linked to an existing issue in this project that this change addresses.
• [x ] I have read the skills.
• [x ] I have read the DESIGN.md file(s) in packages relevent to my changes.
• I have updated the DESIGN.md file(s) in packages relevent to my changes.

⏭ Next Steps

• Add a change file using npm run change if required by the maintainers.
• Update documentation if the declarative f-children filter syntax is intended to be part of the public API.

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[janechu]

We test in browser for declarative format, please check the fixture files for examples of how to do this. In this case you should see fast-element/test/declarative/fixtures/directives/children.

[janechu]

split(" filter ") also splits inside valid CSS selectors. For example, f-children="{items filter elements([data-kind="a filter b"])}" leaves parts[1] as elements([data-kind="a, so lastIndexOf(")") is -1 and this falls back to elements(undefined), matching every element instead of the requested selector. Please parse the filter elements(...) suffix without discarding the rest of the selector (for example, split only on the first directive delimiter and keep the remaining selector text). As more directive modifiers are added, it would also be better to make this a small modifier parser/registry supplied by the directive rather than hard-coding each new option in TemplateParser, which keeps the pure declarative parser from growing with every directive-specific feature.