fix(formatter): preserve parentheses for `in` expressions in arrow function block bodies by Boshen · Pull Request #18352 · oxc-project/oxc

Boshen · 2026-01-21T14:05:13Z

Summary

Fix parentheses not being added for in expressions nested inside arrow function block bodies within ForStatement.init
JS prettier conformance: 737/754 → 738/754 (97.75% → 97.88%)

Before:

for (var a = (() => { b in c; });;);
// formatted to:
for (var a = () => { b in c; }; ; );  // missing parens

After:

for (var a = (() => { b in c; });;);
// formatted to:
for (var a = () => { (b in c); }; ; );  // parens preserved

Root Cause

is_arrow_function_body() only returns true for expression bodies (() => expr) but not block bodies (() => { expr; }). The is_in_for_initializer function was returning false early when encountering an ExpressionStatement inside a block body.

Test plan

cargo run -p oxc_prettier_conformance -- --filter "for/parentheses" now passes
cargo test -p oxc_formatter passes

🤖 Generated with Claude Code

Boshen · 2026-01-21T14:11:52Z

Merge activity

Jan 21, 2:11 PM UTC: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
Jan 21, 2:11 PM UTC: Boshen added this pull request to the Graphite merge queue.
Jan 21, 2:18 PM UTC: Merged by the Graphite merge queue.

…nction block bodies (#18352) ## Summary - Fix parentheses not being added for `in` expressions nested inside arrow function **block bodies** within `ForStatement.init` - JS prettier conformance: 737/754 → 738/754 (97.75% → 97.88%) Before: ```javascript for (var a = (() => { b in c; });;); // formatted to: for (var a = () => { b in c; }; ; ); // missing parens ``` After: ```javascript for (var a = (() => { b in c; });;); // formatted to: for (var a = () => { (b in c); }; ; ); // parens preserved ``` ## Root Cause `is_arrow_function_body()` only returns `true` for expression bodies (`() => expr`) but not block bodies (`() => { expr; }`). The `is_in_for_initializer` function was returning `false` early when encountering an `ExpressionStatement` inside a block body. ## Test plan - `cargo run -p oxc_prettier_conformance -- --filter "for/parentheses"` now passes - `cargo test -p oxc_formatter` passes 🤖 Generated with [Claude Code](https://claude.ai/code)

Copilot

Pull request overview

Fixes missing parentheses insertion for in binary expressions that occur inside arrow-function block bodies when the arrow is nested in ForStatement.init, improving Prettier conformance.

Changes:

Extend is_in_for_initializer ancestor-walk logic to avoid returning false early for ExpressionStatements within an arrow block body.
Update Prettier conformance snapshot to reflect the improved JS compatibility score.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
tasks/prettier_conformance/snapshots/prettier.js.snap.md	Updates the recorded JS compatibility score / failed-spec list after the formatter change.
crates/oxc_formatter/src/parentheses/expression.rs	Adjusts `in`-expression parentheses detection so arrow-function block body statements in `ForStatement.init` are handled.

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

Copilot · 2026-01-21T14:14:11Z

crates/oxc_formatter/src/parentheses/expression.rs

+                // Block body: `() => { expr; }` - check if we're inside a FunctionBody
+                if matches!(stmt.parent, AstNodes::FunctionBody(_)) {
+                    continue;


The new matches!(stmt.parent, AstNodes::FunctionBody(_)) check will be true for ExpressionStatements inside any function body (regular functions/methods as well as arrows), not just arrow-function block bodies. If this change is meant to target arrow functions only, consider narrowing the condition to verify the enclosing FunctionBody belongs to an ArrowFunctionExpression (and optionally arrow.expression == false) to avoid changing parentheses behavior in other function bodies nested under ForStatement.init.

Suggested change

// Block body: `() => { expr; }` - check if we're inside a FunctionBody

if matches!(stmt.parent, AstNodes::FunctionBody(_)) {

continue;

// Block body: `() => { expr; }` - check if we're inside an arrow function's FunctionBody

if let AstNodes::FunctionBody(body) = stmt.parent {

if let AstNodes::ArrowFunctionExpression(arrow) = body.parent {

// Only treat block-body arrow functions specially (`() => { ... }`)

if !arrow.expression {

continue;

}

}

Copilot · 2026-01-21T14:14:12Z

crates/oxc_formatter/src/parentheses/expression.rs

+            // FunctionBody: Continue checking - could be inside arrow function in ForStatement.init
            AstNodes::ForStatement(stmt) => {


This comment mentions FunctionBody, but it’s placed on the ForStatement match arm. Consider moving/rewording it so it documents the intended behavior (e.g., that FunctionBody is intentionally ignored via the default arm) rather than implying this arm handles FunctionBody.

Copilot · 2026-01-21T14:14:12Z

crates/oxc_formatter/src/parentheses/expression.rs

+                // Block body: `() => { expr; }` - check if we're inside a FunctionBody
+                if matches!(stmt.parent, AstNodes::FunctionBody(_)) {
+                    continue;
+                }


The new behavior (treating in expressions inside arrow-function block bodies as requiring parentheses when nested under ForStatement.init) doesn’t appear to have a dedicated formatter fixture regression test. Please add a small crates/oxc_formatter/tests/fixtures/js case covering an arrow with a block body inside for (...;...;...) where the block contains an in expression statement, so future refactors to is_in_for_initializer don’t regress this again.

# Oxlint ### 💥 BREAKING CHANGES - 777fc40 ast: [**BREAKING**] Add `Ident` type (#18354) (Boshen) ### 🚀 Features - 34c3ec3 linter/prefer-logical-operator-over-ternary: Implement fixer (#18545) (camc314) - 019e0aa linter/valid-typeof: Add suggestions if type is misspelled (#18543) (camchenry) - 704c8eb linter/use-isnan: Add more specific error message for equality/inequality (#18542) (camchenry) - 1e99ace linter/use-isnan: Support more `indexOf` cases and improve diagnostic messages (#18537) (camchenry) - bffd134 linter/text-encoding-identifier-case: Add `withDash` option (#18533) (camc314) - 993fd2b parser: Parse unambiguous await with better error messages (#18480) (Boshen) - b4b6247 linter/plugins: `RuleTester` support settings (#18445) (overlookmotel) - 15d69dc linter: Implement react/display-name rule (#18426) (camchenry) - 2fbceae linter: Implement rule docs and config support for rules with tuple config options. (#18372) (connorshea) - 8db0e78 linter/plugins: Handle BOMs (#18376) (overlookmotel) - 6ac09e2 linter/plugins: Support source text not being at start of buffer (#18375) (overlookmotel) - fc3c86b linter: Update 125 rules to raise errors when provided with invalid config options. (#18104) (connorshea) - 2cc6ad2 linter/plugins: Add `ecmaFeatures` to `parserOptions` (#18313) (overlookmotel) ### 🐛 Bug Fixes - 2acf568 linter/plugins: Keep `Infinity` in rule default options (#18550) (overlookmotel) - 332d2ef linter/plugins: Add `jsx` property to `parserOptions.ecmaFeatures` (#18549) (overlookmotel) - 7d9bb1b linter: Update `eslint/func-names` to error on invalid rule config options, improve docs. (#18510) (connorshea) - 9c67974 linter: Improve the jsx-a11y/no-noninteractive-tabindex rule to match original rule logic better (#17848) (connorshea) - 75e7163 vscode: Support json5 for oxfmt (#18502) (Sysix) - c205b0d ast: Remove `ThisExpression` from `TSModuleReference` (#18489) (Boshen) - c51339a oxlint/lsp: Respect code action `source.fixAll` as an alias for `source.fixAll.oxc` (#18366) (Sysix) - 3c0e9b9 oxlint/lsp: Skip dangerous fixes/suggestions for "fix all" code action and command (#18364) (Sysix) - c44c093 linter: Fix behavior of unicorn/catch-error-name to match original rule (#18209) (connorshea) - 9c65aff linter/jsx-a11y: Change `no-autofocus` autofix to suggestion (#18155) (Ben Lowery) - 235c820 linter/unicorn: Fix `prefer-array-some` autofix for `.filter().length` pattern (#18153) (Ben Lowery) - a9925dc linter: Mark fixes in `unicorn/no-null` rule as dangerous. (#18436) (connorshea) - cee29b4 linter: Remove confusing scope from `react/only-export-components` rule diagnostics. (#18434) (connorshea) - aed3669 parser: Parse HTML-like comments in unambiguous mode (#18442) (Boshen) - b8a371d linter: Fix the path used in the gitlab format output (#18165) (connorshea) - e046ea6 linter: `vue/no-lifecycle-after-await` skip looking into arrow functions (#18302) (Sysix) - a9bfbcf linter: Compatibility issue with `DiagnosticData` type in ESLint (#18396) (루밀LuMir) - 10ab424 linter: `react/no_array_index_key` continue search for other attributes (#18409) (Lonami) - 9d776d4 linter: Update `import/no-cycle` rule to error on invalid config options. (#18330) (connorshea) - c163231 linter: Update eslint/sort-imports to validate options. (#18378) (connorshea) - 79bbcff linter: Update `eslint/func-style` to error on invalid configuration options. (#18390) (connorshea) - b871235 linter/plugins: Fix identifying "use strict" directives in scope analysis (#18402) (overlookmotel) - 5985141 linter: Update `jest/prefer-lowercase-title` rule to error on invalid config options. (#18332) (connorshea) - faca4b5 linter/plugins: Tokenize `let`, `static` and `yield` as `Keyword`s (#18368) (overlookmotel) - a3914fd linter/plugins: Allow line number passed to `report` to be 1 over line count (#18341) (overlookmotel) - 88e0896 linter: Update `typescript/no-restricted-types` rule to error on invalid config options. (#18329) (connorshea) - 9eec600 linter: Update `react/jsx-fragments` rule to raise an error on invalid configuration options (#18111) (connorshea) - 0fa969d linter: Update `react/no-will-update-set-state` to error on invalid config options (#18112) (connorshea) - 70e7be4 linter: Update `import/no-unassigned-import` to raise an error when passed invalid config options. (#18108) (connorshea) - 496cac7 linter: Update `unicorn/explicit-length-check` to raise an error when passed invalid config options. (#18107) (connorshea) - 080b1ec linter: Update 5 more rules to error on invalid config options. (#18113) (connorshea) - c5d05dd linter: Update 11 rules to raise an error on invalid config options. (#18109) (connorshea) - 9e359d4 linter/plugins: Set all properties on global vars objects (#18317) (overlookmotel) - 39c7f32 linter/plugins: Set `writeable` flag on variables where defined as globals (#18316) (overlookmotel) - a570693 linter/plugins: Fix `CatchClause` scopes (#18312) (overlookmotel) - 8c98e69 linter: `vitest/prefer-describe-function-title`: Check earlier to avoid false positive (#18177) (Jovi De Croock) - 44be0eb linter/plugins: Set scope analyse settings based on source type (#18306) (overlookmotel) - b9a14fd vscode: Update package.json to restrict a few more config options. (#18270) (Connor Shea) - c1260cb vscode: Update version info formatting. (#18274) (connorshea) - 2f68dc6 vscode: Update notification for client restart to specify tool. (#18273) (connorshea) ### ⚡ Performance - dc931ba linter/no-inner-declarations: Skip scope flags lookup in modules (#18249) (overlookmotel) - 07618a7 linter: Turn off `scope_build_child_ids` for SemanticBuilder (#18360) (Dunqing) - 1aac079 linter/exhaustive-deps: Simplify the logic of checking if the identifier it is a dependency of hook (#18350) (Dunqing) - 591d522 linter/block-scoped-var: Avoid `iter_all_scope_child_ids` by walking references/redeclarations scope ancestors (#18335) (Dunqing) - 2eefd6d linter/plugins: Remove branch from token parsing (#18369) (overlookmotel) ### 📚 Documentation - 698c21d linter: Modernize docs for various React rules (#18559) (connorshea) - 314a47c linter: Clarify the `no-find-dom-node` rule with a note that the method was removed in React 19. (#18556) (connorshea) - 5eff704 linter: Update `no-inner-declarations` to fix config option docs (#18511) (connorshea) - dd5d2f6 linter: Improve diagnostic message in `valid_typeof` rule. (#18507) (connorshea) - 8ccd853 npm: Update package homepage URLs and add keywords (#18509) (Boshen) - 4958233 linter: Add missing "What it does" section in prefer-reflect-apply rule. (#18475) (connorshea) - 2fa83a4 linter: Improve the docs for import/unambiguous. (#18474) (connorshea) - 7b1505c linter: Improve docs for `oxc/only-used-in-recursion` rule. (#18473) (connorshea) - ab506d6 linter/plugins: Correct comment (#18456) (overlookmotel) - 4565c73 linter: `react/display-name`: add docs for config options (#18430) (camchenry) - b95a89f linter: Fix docs for the curly rule. (#18374) (connorshea) - f675eb4 linter: Fix the `react/only-export-components` rule docs. (#18319) (connorshea) - 704db95 linter: "no-unused-vars" extend ignored files section for svelte and astro files (#18304) (Sysix) - 3af4a88 linter: Add "Examples" headers to rules missing them (#18266) (connorshea) # Oxfmt ### 💥 BREAKING CHANGES - 777fc40 ast: [**BREAKING**] Add `Ident` type (#18354) (Boshen) ### 🚀 Features - d71c15d oxfmt: Enable tailwind sort inside xxx-in-js (#18417) (leaysgur) - 52b5003 formatter,oxfmt: Support Angular `@Component({ template, styles })` (#18324) (leaysgur) ### 🐛 Bug Fixes - 224140c oxfmt: Canonicalize `..` component in config path (#18570) (leaysgur) - 30b467e formatter: Preserve trailing comments before the semicolon in class methods without a body (#18446) (Dunqing) - c205b0d ast: Remove `ThisExpression` from `TSModuleReference` (#18489) (Boshen) - 164bbd7 formatter: Preserve trailing comments inside ternary alternate branch (#18433) (Dunqing) - 1c50800 formatter: Use HTML entity escaping for JSX attribute strings (#18385) (Boshen) - 4e156d2 formatter: Preserve parentheses for `in` expressions in arrow function block bodies (#18352) (Boshen) - 7e6c15b oxfmt: Increase Tailwind CSS test timeout for Windows CI (#18339) (Boshen) - 29966eb formatter/dead-code-removal: Handle tailwind sorting (#18321) (leaysgur) - 29f41be formatter: Only expand mapped types when newline immediately follows opening brace (#18087) (Boshen) - 2194552 formatter: Relocate leading comments for single-element union/intersection types (#18083) (Boshen) ### ⚡ Performance - 85ab400 formatter: Store `AstNodes` itself instead of `&'a AstNodes` as the `parent` field of `AstNode` (#18428) (Dunqing) - 194d384 formatter: Reduce AstNode size by 8 bytes using following_span_start (#18347) (Dunqing) - b2df8fb oxfmt: Enable tailwind plugin only for relevant parser (#18418) (leaysgur) ### 📚 Documentation - 8ccd853 npm: Update package homepage URLs and add keywords (#18509) (Boshen) Co-authored-by: Boshen <[email protected]>

Boshen requested a review from Dunqing as a code owner January 21, 2026 14:05

Copilot AI review requested due to automatic review settings January 21, 2026 14:05

github-actions bot added A-formatter Area - Formatter C-bug Category - Bug labels Jan 21, 2026

Copilot started reviewing on behalf of Boshen January 21, 2026 14:05 View session

Boshen added the 0-merge Merge with Graphite Merge Queue label Jan 21, 2026

graphite-app bot force-pushed the fix/formatter-in-parens-arrow-body branch from 0be3bed to 4e156d2 Compare January 21, 2026 14:12

Copilot AI reviewed Jan 21, 2026

View reviewed changes

graphite-app bot merged commit 4e156d2 into main Jan 21, 2026
21 checks passed

graphite-app bot deleted the fix/formatter-in-parens-arrow-body branch January 21, 2026 14:18

graphite-app bot removed the 0-merge Merge with Graphite Merge Queue label Jan 21, 2026

Boshen mentioned this pull request Jan 26, 2026

release(apps): oxlint v1.42.0 && oxfmt v0.27.0 #18573

Merged

Provide feedback

Saved searches

Use saved searches to filter your results more quickly

Uh oh!

fix(formatter): preserve parentheses for `in` expressions in arrow function block bodies#18352

fix(formatter): preserve parentheses for `in` expressions in arrow function block bodies#18352
graphite-app[bot] merged 1 commit intomainfrom
fix/formatter-in-parens-arrow-body

Boshen commented Jan 21, 2026

Uh oh!

Boshen commented Jan 21, 2026 •

edited by graphite-app bot

Loading

Uh oh!

Copilot AI left a comment

Uh oh!

Copilot AI Jan 21, 2026

Uh oh!

Copilot AI Jan 21, 2026

Uh oh!

Copilot AI Jan 21, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

1 participant

Comments

-                // Block body: `() => { expr; }` - check if we're inside a FunctionBody
-                if matches!(stmt.parent, AstNodes::FunctionBody(_)) {
-                    continue;
+                // Block body: `() => { expr; }` - check if we're inside an arrow function's FunctionBody
+                if let AstNodes::FunctionBody(body) = stmt.parent {
+                    if let AstNodes::ArrowFunctionExpression(arrow) = body.parent {
+                        // Only treat block-body arrow functions specially (`() => { ... }`)
+                        if !arrow.expression {
+                            continue;
+                        }
+                    }

		// FunctionBody: Continue checking - could be inside arrow function in ForStatement.init
		AstNodes::ForStatement(stmt) => {

Uh oh!

Conversation

Boshen commented Jan 21, 2026

Summary

Root Cause

Test plan

Uh oh!

Boshen commented Jan 21, 2026 • edited by graphite-app bot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Merge activity

Uh oh!

Copilot AI left a comment

Choose a reason for hiding this comment

Pull request overview

Reviewed changes

Uh oh!

Copilot AI Jan 21, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 21, 2026

Choose a reason for hiding this comment

Uh oh!

Copilot AI Jan 21, 2026

Choose a reason for hiding this comment

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

1 participant

Comments

Boshen commented Jan 21, 2026 •

edited by graphite-app bot

Loading