fix(linter): noUselessUndefinedInitialization false positive in svelte 4.

[codiini]

I used Claude Code to think through the issue and validate my solution with tests.

Summary

Fixes #6003

The noUselessUndefinedInitialization rule was producing false positives for exported variables. In Svelte 4, exported variables with undefined initialization are used to declare optional component props:

​js let className = undefined; export { className as class }; ​

The rule incorrectly flagged this pattern because it only looked at the variable declaration in isolation, without considering that it would be exported later.

Solution

Changed the rule to use Biome's semantic model to check if a variable is exported before flagging it. This handles all export patterns:

• Direct exports: export let x = undefined;
• Named exports: let x = undefined; export { x };
• Renamed exports: let x = undefined; export { x as y };
• Default exports: let x = undefined; export default x;

Test Plan

• Added test cases for all export patterns in noUselessUndefinedInitialization/valid.js
• Ran cargo test -p biome_js_analyze --test spec_tests -- specs::complexity::no_useless_undefined_initialization - all tests pass
• Manually verified with the exact pattern from the issue:
  • Before fix; undefined is incorrectly removed
  • After fix; undefined is preserved for exported variables

Docs

Documentation is included in the rule's rustdoc comments with examples showing that exported variables are not flagged.

[changeset-bot]

🦋 Changeset detected

Latest commit: 9dd6a3a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 13 packages

Name	Type
@biomejs/biome	Patch
@biomejs/cli-win32-x64	Patch
@biomejs/cli-win32-arm64	Patch
@biomejs/cli-darwin-x64	Patch
@biomejs/cli-darwin-arm64	Patch
@biomejs/cli-linux-x64	Patch
@biomejs/cli-linux-arm64	Patch
@biomejs/cli-linux-x64-musl	Patch
@biomejs/cli-linux-arm64-musl	Patch
@biomejs/wasm-web	Patch
@biomejs/wasm-bundler	Patch
@biomejs/wasm-nodejs	Patch
@biomejs/backend-jsonrpc	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[coderabbitai]

Walkthrough

Adds a changeset entry and updates the noUselessUndefinedInitialization lint to use semantic analysis (Semantic) and the project's semantic model to detect exported bindings. The rule now skips variables initialized to undefined when they are exported. Tests were added/updated to include direct exports, named/renamed exports, and a default-export pattern where variables are initialised to undefined.

Suggested reviewers

• dyc3

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: fixing a false positive in the noUselessUndefinedInitialization rule for Svelte 4 exported variables.
Description check	✅ Passed	The description is well-structured and directly related to the changeset, explaining the problem, solution, test plan, and documentation updates.
Linked Issues check	✅ Passed	The PR fully addresses issue #6003: it detects all export patterns (direct, named, renamed, default) and prevents false positives for exported variables initialised to undefined.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the noUselessUndefinedInitialization rule behaviour and its test suite; no extraneous modifications detected.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing touches

• 📝 Generate docstrings

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[dyc3]

Just a nitpick: "Svelte" -> "Svelte 4", because Svelte 5 declares props differently (even though the non-runes version is still supported)

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing codiini:fix/biome-6003-no-useless-undefined (9dd6a3a) with main (96d09f4)}

Summary

✅ 58 untouched benchmarks
⏩ 95 skipped benchmarks¹

Footnotes

1. 95 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[ematipico]

Lol, the output emitted in the PR description is bigger than the code itself

[dyc3]

@codiini You'll need to update the snapshots

[codiini]

@dyc3 All done.

[ematipico]

I think it's safe to land it as a patch, since we're adding a check to an edge case