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

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Merged
graphite-app merged 1 commit into from
Jan 21, 2026
Merged

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

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6 changes: 6 additions & 0 deletions crates/oxc_formatter/src/parentheses/expression.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -406,14 +406,20 @@ fn is_in_for_initializer(expr: &AstNode<'_, BinaryExpression<'_>>) -> bool {
match parent {
AstNodes::ExpressionStatement(stmt) => {
if stmt.is_arrow_function_body() {
// Expression body: `() => expr`
// Skip `FunctionBody` and `ArrowFunctionExpression`
let skipped = ancestors.by_ref().nth(1);
debug_assert!(matches!(skipped, Some(AstNodes::ArrowFunctionExpression(_))));
continue;
}
// Block body: `() => { expr; }` - check if we're inside a FunctionBody
if matches!(stmt.parent, AstNodes::FunctionBody(_)) {
continue;
Comment on lines +415 to +417
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Copilot AI Jan 21, 2026

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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;
}
}

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}
Comment on lines +415 to +418
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Copilot AI Jan 21, 2026

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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.

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return false;
}
// FunctionBody: Continue checking - could be inside arrow function in ForStatement.init
AstNodes::ForStatement(stmt) => {
Comment on lines +422 to 423
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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.

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return stmt
.init
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5 changes: 2 additions & 3 deletions tasks/prettier_conformance/snapshots/prettier.js.snap.md
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Original file line number Diff line number Diff line change
@@ -1,4 +1,4 @@
js compatibility: 737/754 (97.75%)
js compatibility: 738/754 (97.88%)

# Failed

Expand All @@ -11,8 +11,7 @@ js compatibility: 737/754 (97.75%)
| js/comments/return-statement.js | 💥💥 | 98.85% |
| js/explicit-resource-management/valid-await-using-comments.js | 💥 | 80.00% |
| js/for/9812-unstable.js | 💥 | 63.64% |
| js/for/for-in-with-initializer.js | 💥 | 37.50% |
| js/for/parentheses.js | 💥 | 97.96% |
| js/for/for-in-with-initializer.js | 💥 | 31.25% |
| js/last-argument-expansion/dangling-comment-in-arrow-function.js | 💥 | 22.22% |
| js/quote-props/objects.js | 💥💥✨✨ | 48.04% |
| js/quote-props/with_numbers.js | 💥💥✨✨ | 46.43% |
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