fix(linter): prevent useConsistentArrowReturn from producing ASI-affected code #8440

tt-a1i · 2025-12-14T01:40:40Z

Summary

Fixed #8179: useConsistentArrowReturn autofix no longer produces semantically incorrect code when adding braces to arrow functions with multiline expressions.

Problem

When running biome lint --fix with useConsistentArrowReturn configured with style: "always", the autofix could produce code that silently changes runtime behavior:

Input:

const foo = (l: string) =>
  l
    .split('\n')

Before (incorrect autofix):

const foo = (l: string) =>
  {
  return
  l
    .split("\n");
}

The function now returns undefined instead of the split array because JavaScript's automatic semicolon insertion (ASI) inserts a semicolon after return when followed by a newline.

Solution

Strip leading trivia (newlines/whitespace) from the expression before inserting it after the return keyword, ensuring return and the expression stay on the same line.

After (correct autofix):

const foo = (l: string) =>
  {
  return l
    .split("\n");
}

Changes

use_consistent_arrow_return.rs: Added trim_leading_trivia() call on the expression
Added test cases for multiline expressions

AI Assistance Disclosure

This PR was written with assistance from Claude Code.

…cted code When adding braces to arrow functions with multiline expressions, the autofix previously preserved leading trivia (newlines) from the original expression. This could place a newline between `return` and the expression, triggering JavaScript's automatic semicolon insertion and causing the function to return `undefined` instead of the value. Fixed by trimming leading trivia from the expression before inserting it after the `return` keyword. Fixes biomejs#8179

changeset-bot · 2025-12-14T01:40:45Z

🦋 Changeset detected

Latest commit: e4bc3f2

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

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coderabbitai · 2025-12-14T01:43:02Z

Walkthrough

This PR fixes a critical bug in the useConsistentArrowReturn rule's autofix where adding braces to arrow functions inadvertently inserted a newline between return and the expression. This triggered JavaScript's automatic semicolon insertion (ASI), causing functions to return undefined instead of their intended values. The fix trims leading trivia from the expression before constructing the return statement to prevent this issue. Test cases are added to validate handling of multiline expressions.

Possibly related PRs

feat(biome_js_analyze): add UseConsistentArrowReturn rule #7245: Modifies the same UseConsistentArrowReturn rule implementation file with overlapping logic for return statement construction.

Suggested labels

A-Linter, L-JavaScript

Suggested reviewers

arendjr
siketyan
ematipico

Pre-merge checks and finishing touches

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately describes the main fix: preventing useConsistentArrowReturn from producing code affected by automatic semicolon insertion.
Description check	✅ Passed	The description clearly explains the bug, the problem with ASI, the solution (trimming leading trivia), and the expected output.
Linked Issues check	✅ Passed	The PR addresses all requirements from issue #8179: fixing the autofix to not produce ASI-affected code, ensuring correct semantics, and adding test cases for multiline expressions.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the useConsistentArrowReturn autofix issue—the rule implementation, changeset entry, and test cases are all relevant.

✨ Finishing touches

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Create PR with unit tests
Post copyable unit tests in a comment

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Actionable comments posted: 0

🧹 Nitpick comments (2)

crates/biome_js_analyze/src/lint/nursery/use_consistent_arrow_return.rs (1)

182-186: Good fix: trim leading trivia to avoid return + newline ASI traps.

This looks like the right surgical move: trim_leading_trivia() ensures the returned expression starts immediately after return , preserving runtime semantics for multiline arrow bodies. Nice.
Consider adding a one-liner to the rule rustdoc noting the autofix avoids ASI pitfalls for multiline expressions (since this is a behavioural fix). As per coding guidelines, ...

.changeset/fix-use-consistent-arrow-return-asi.md (1)

1-5: Changeset: please add a tiny before/after code example for the rule.

Guidelines for .changeset/*.md ask for a code example; a minimal () =>\n foo.bar() → () => { return foo.bar(); } style snippet would make this much easier to grok at release-note speed. As per coding guidelines, ...

📜 Review details

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📥 Commits

Reviewing files that changed from the base of the PR and between a392c06 and e4bc3f2.

⛔ Files ignored due to path filters (1)

crates/biome_js_analyze/tests/specs/nursery/useConsistentArrowReturn/always.invalid.js.snap is excluded by !**/*.snap and included by **

📒 Files selected for processing (3)

.changeset/fix-use-consistent-arrow-return-asi.md (1 hunks)
crates/biome_js_analyze/src/lint/nursery/use_consistent_arrow_return.rs (1 hunks)
crates/biome_js_analyze/tests/specs/nursery/useConsistentArrowReturn/always.invalid.js (1 hunks)

🧰 Additional context used

📓 Path-based instructions (2)

.changeset/*.md

📄 CodeRabbit inference engine (CONTRIBUTING.md)

Write changesets that are concise (1-3 sentences), user-focused, use past tense for actions taken and present tense for Biome behavior, include code examples for rules, and end sentences with periods

Files:

.changeset/fix-use-consistent-arrow-return-asi.md

crates/**/*.rs

📄 CodeRabbit inference engine (CONTRIBUTING.md)

Update inline rustdoc documentation for rules, assists, and their options when adding new features or changing existing features in Rust crates

Files:

crates/biome_js_analyze/src/lint/nursery/use_consistent_arrow_return.rs

🧠 Learnings (12)

📓 Common learnings

Learnt from: arendjr
Repo: biomejs/biome PR: 7593
File: crates/biome_service/src/workspace/server.rs:1306-1306
Timestamp: 2025-09-25T12:32:59.003Z
Learning: In the biomejs/biome project, do not flag compilation errors during code review as they are handled by the existing test infrastructure and CI. Focus on other code quality aspects instead.

📚 Learning: 2025-12-12T10:11:05.549Z

Learnt from: CR
Repo: biomejs/biome PR: 0
File: CONTRIBUTING.md:0-0
Timestamp: 2025-12-12T10:11:05.549Z
Learning: Applies to .changeset/*.md : Write changesets that are concise (1-3 sentences), user-focused, use past tense for actions taken and present tense for Biome behavior, include code examples for rules, and end sentences with periods

Applied to files:

.changeset/fix-use-consistent-arrow-return-asi.md