From a5314a1c8a01e06ed6717902121ecbd76d54902c Mon Sep 17 00:00:00 2001
From: leaysgur <6259812+leaysgur@users.noreply.github.com>
Date: Mon, 26 Oct 2026 05:43:53 +0000
Subject: [PATCH] fix(formatter): keep `;` before `in` and `instanceof` class
 members (#28166)

Closes #28065

Prettier's `shouldPrintSemicolonAfterClassProperty` (and also biome) already has this rule, but our port missed it.

- Treat an `in` / `instanceof` key like a computed key in `needs_semicolon`, with the same modifier exclusions
- Check a string key by its value, since `"in"` may be printed unquoted
  - Prettier unquotes it and drops the `;`, producing invalid code
  - DIVERGENCES.md#string-binary-keyword-class-key
---
 crates/oxc_formatter/DIVERGENCES.md           |  29 ++++
 crates/oxc_formatter/src/print/class.rs       |  27 +++-
 .../js/semicolons/binary-keyword-class-key.js |  22 +++
 .../binary-keyword-class-key.js.snap          | 133 ++++++++++++++++++
 .../binary-keyword-class-key-quoted.ts        |   5 +
 .../binary-keyword-class-key-quoted.ts.snap   |  48 +++++++
 tasks/coverage/snapshots/formatter_babel.snap |   6 +-
 7 files changed, 261 insertions(+), 9 deletions(-)
 create mode 100644 crates/oxc_formatter/tests/fixtures/js/semicolons/binary-keyword-class-key.js
 create mode 100644 crates/oxc_formatter/tests/fixtures/js/semicolons/binary-keyword-class-key.js.snap
 create mode 100644 crates/oxc_formatter/tests/fixtures/ts/semicolons/binary-keyword-class-key-quoted.ts
 create mode 100644 crates/oxc_formatter/tests/fixtures/ts/semicolons/binary-keyword-class-key-quoted.ts.snap

diff --git a/crates/oxc_formatter/DIVERGENCES.md b/crates/oxc_formatter/DIVERGENCES.md
index ff0b581db0..e8ab86276e 100644
--- a/crates/oxc_formatter/DIVERGENCES.md
+++ b/crates/oxc_formatter/DIVERGENCES.md
@@ -1050,3 +1050,32 @@ const --> comment
 A closing HTML comment (`-->`, Annex B) is a comment only at the start of a line.
 Prettier inlines it after `const` or `var`, producing code rejected by `node --check`, and misparses the `let` case as an expression statement.
 We always print a line break before `-->`, which also splits a block comment preceding it on the same line, where Prettier's output stays valid.
+
+## string-binary-keyword-class-key
+
+- Why: semantics
+- Pin: `tests/fixtures/js/semicolons/binary-keyword-class-key.js`, `tests/fixtures/ts/semicolons/binary-keyword-class-key-quoted.ts`
+
+```js
+// input (semi: false)
+class A {
+  a = 1;
+  "in";
+}
+
+// ours
+class A {
+  a = 1;
+  in
+}
+
+// prettier
+class A {
+  a = 1
+  in
+}
+```
+
+A class member named `in` or `instanceof` would continue the previous property's value as a binary operator, so the `;` before it is kept.
+Prettier checks only an identifier key, but also prints the string key `"in"` unquoted, producing code rejected by `node --check`.
+We check a string key by its value, which also keeps the `;` before a key that stays quoted (TypeScript, `quoteProps: "preserve"`), where Prettier's output stays valid.
diff --git a/crates/oxc_formatter/src/print/class.rs b/crates/oxc_formatter/src/print/class.rs
index c94faab07f..1699b0258a 100644
--- a/crates/oxc_formatter/src/print/class.rs
+++ b/crates/oxc_formatter/src/print/class.rs
@@ -571,10 +571,15 @@ impl<'a, 'b> FormatClassElementWithSemicolon<'a, 'b> {
 
         let Some(next_element) = next_element else { return false };
 
+        // When the name starts with `[`, `in` or `instanceof`.
+        // NOTE: These only continue a previous property with a value (`x = a` + `in`, `x = a` + `[b]`),
+        // but kept regardless of it like Prettier, an unneeded `;` is harmless.
+        let starts_with_hazard = next_element.computed()
+            || next_element.property_key().is_some_and(is_binary_keyword_key);
         match next_element.as_ref() {
-            // When the name starts with the generator token or `[`
+            // Or the generator token
             ClassElement::MethodDefinition(def) if !def.value.r#async => {
-                (def.computed
+                (starts_with_hazard
                     && !(def.kind.is_accessor()
                         || def.r#static
                         || def.accessibility.is_some()
@@ -582,7 +587,7 @@ impl<'a, 'b> FormatClassElementWithSemicolon<'a, 'b> {
                     || def.value.generator
             }
             ClassElement::PropertyDefinition(def) => {
-                def.computed
+                starts_with_hazard
                     && !(def.accessibility.is_some()
                         || def.r#static
                         || def.declare
@@ -590,7 +595,8 @@ impl<'a, 'b> FormatClassElementWithSemicolon<'a, 'b> {
                         || def.readonly)
             }
             ClassElement::AccessorProperty(def) => {
-                def.computed && !(def.accessibility.is_some() || def.r#static || def.r#override)
+                starts_with_hazard
+                    && !(def.accessibility.is_some() || def.r#static || def.r#override)
             }
             ClassElement::TSIndexSignature(_) => true,
             _ => false,
@@ -598,6 +604,19 @@ impl<'a, 'b> FormatClassElementWithSemicolon<'a, 'b> {
     }
 }
 
+/// `in` or `instanceof` would continue the previous property's value as a binary operator.
+/// A string key may be printed unquoted, so it is checked by its value.
+/// See DIVERGENCES.md#string-binary-keyword-class-key
+fn is_binary_keyword_key(key: &PropertyKey<'_>) -> bool {
+    match key {
+        PropertyKey::StaticIdentifier(ident) => matches!(ident.name.as_str(), "in" | "instanceof"),
+        PropertyKey::StringLiteral(literal) => {
+            literal.value == "in" || literal.value == "instanceof"
+        }
+        _ => false,
+    }
+}
+
 impl FormatClassElementWithSemicolon<'_, '_> {
     /// Where the element's content ends and its `;` takes over, for the kinds that take one:
     /// a property's value (the chain leaf, like an expression statement), type annotation or key,
diff --git a/crates/oxc_formatter/tests/fixtures/js/semicolons/binary-keyword-class-key.js b/crates/oxc_formatter/tests/fixtures/js/semicolons/binary-keyword-class-key.js
new file mode 100644
index 0000000000..27cbf08ef3
--- /dev/null
+++ b/crates/oxc_formatter/tests/fixtures/js/semicolons/binary-keyword-class-key.js
@@ -0,0 +1,22 @@
+// DIVERGENCES.md#string-binary-keyword-class-key
+class A {
+  a = 1;
+  in;
+  b = 1;
+  instanceof() {}
+  c;
+  in;
+  d = 1;
+  "in";
+  e = 1;
+  "instanceof"() {}
+}
+
+class B {
+  a = 1;
+  static in;
+  b = 1;
+  get in() {}
+  c = 1;
+  async in() {}
+}
diff --git a/crates/oxc_formatter/tests/fixtures/js/semicolons/binary-keyword-class-key.js.snap b/crates/oxc_formatter/tests/fixtures/js/semicolons/binary-keyword-class-key.js.snap
new file mode 100644
index 0000000000..b37dfd0bf8
--- /dev/null
+++ b/crates/oxc_formatter/tests/fixtures/js/semicolons/binary-keyword-class-key.js.snap
@@ -0,0 +1,133 @@
+---
+source: crates/oxc_formatter/tests/fixtures/mod.rs
+---
+==================== Input ====================
+// DIVERGENCES.md#string-binary-keyword-class-key
+class A {
+  a = 1;
+  in;
+  b = 1;
+  instanceof() {}
+  c;
+  in;
+  d = 1;
+  "in";
+  e = 1;
+  "instanceof"() {}
+}
+
+class B {
+  a = 1;
+  static in;
+  b = 1;
+  get in() {}
+  c = 1;
+  async in() {}
+}
+
+==================== Output ====================
+------------------------------
+{ printWidth: 80, semi: true }
+------------------------------
+// DIVERGENCES.md#string-binary-keyword-class-key
+class A {
+  a = 1;
+  in;
+  b = 1;
+  instanceof() {}
+  c;
+  in;
+  d = 1;
+  in;
+  e = 1;
+  instanceof() {}
+}
+
+class B {
+  a = 1;
+  static in;
+  b = 1;
+  get in() {}
+  c = 1;
+  async in() {}
+}
+
+-------------------------------
+{ printWidth: 100, semi: true }
+-------------------------------
+// DIVERGENCES.md#string-binary-keyword-class-key
+class A {
+  a = 1;
+  in;
+  b = 1;
+  instanceof() {}
+  c;
+  in;
+  d = 1;
+  in;
+  e = 1;
+  instanceof() {}
+}
+
+class B {
+  a = 1;
+  static in;
+  b = 1;
+  get in() {}
+  c = 1;
+  async in() {}
+}
+
+-------------------------------
+{ printWidth: 80, semi: false }
+-------------------------------
+// DIVERGENCES.md#string-binary-keyword-class-key
+class A {
+  a = 1;
+  in
+  b = 1;
+  instanceof() {}
+  c;
+  in
+  d = 1;
+  in
+  e = 1;
+  instanceof() {}
+}
+
+class B {
+  a = 1
+  static in
+  b = 1
+  get in() {}
+  c = 1
+  async in() {}
+}
+
+--------------------------------
+{ printWidth: 100, semi: false }
+--------------------------------
+// DIVERGENCES.md#string-binary-keyword-class-key
+class A {
+  a = 1;
+  in
+  b = 1;
+  instanceof() {}
+  c;
+  in
+  d = 1;
+  in
+  e = 1;
+  instanceof() {}
+}
+
+class B {
+  a = 1
+  static in
+  b = 1
+  get in() {}
+  c = 1
+  async in() {}
+}
+
+===================== End =====================
diff --git a/crates/oxc_formatter/tests/fixtures/ts/semicolons/binary-keyword-class-key-quoted.ts b/crates/oxc_formatter/tests/fixtures/ts/semicolons/binary-keyword-class-key-quoted.ts
new file mode 100644
index 0000000000..98ef199e21
--- /dev/null
+++ b/crates/oxc_formatter/tests/fixtures/ts/semicolons/binary-keyword-class-key-quoted.ts
@@ -0,0 +1,5 @@
+// DIVERGENCES.md#string-binary-keyword-class-key
+class A {
+  a = 1;
+  "in": number;
+}
diff --git a/crates/oxc_formatter/tests/fixtures/ts/semicolons/binary-keyword-class-key-quoted.ts.snap b/crates/oxc_formatter/tests/fixtures/ts/semicolons/binary-keyword-class-key-quoted.ts.snap
new file mode 100644
index 0000000000..df1b2d8626
--- /dev/null
+++ b/crates/oxc_formatter/tests/fixtures/ts/semicolons/binary-keyword-class-key-quoted.ts.snap
@@ -0,0 +1,48 @@
+---
+source: crates/oxc_formatter/tests/fixtures/mod.rs
+---
+==================== Input ====================
+// DIVERGENCES.md#string-binary-keyword-class-key
+class A {
+  a = 1;
+  "in": number;
+}
+
+==================== Output ====================
+------------------------------
+{ printWidth: 80, semi: true }
+------------------------------
+// DIVERGENCES.md#string-binary-keyword-class-key
+class A {
+  a = 1;
+  "in": number;
+}
+
+-------------------------------
+{ printWidth: 100, semi: true }
+-------------------------------
+// DIVERGENCES.md#string-binary-keyword-class-key
+class A {
+  a = 1;
+  "in": number;
+}
+
+-------------------------------
+{ printWidth: 80, semi: false }
+-------------------------------
+// DIVERGENCES.md#string-binary-keyword-class-key
+class A {
+  a = 1;
+  "in": number
+}
+
+--------------------------------
+{ printWidth: 100, semi: false }
+--------------------------------
+// DIVERGENCES.md#string-binary-keyword-class-key
+class A {
+  a = 1;
+  "in": number
+}
+
+===================== End =====================
diff --git a/tasks/coverage/snapshots/formatter_babel.snap b/tasks/coverage/snapshots/formatter_babel.snap
index 33027eeecb..c8c9702a72 100644
--- a/tasks/coverage/snapshots/formatter_babel.snap
+++ b/tasks/coverage/snapshots/formatter_babel.snap
@@ -2,7 +2,7 @@ commit: a56fe4a4
 
 formatter_babel Summary:
 AST Parsed     : 2280/2280 (100.00%)
-Positive Passed: 2272/2280 (99.65%)
+Positive Passed: 2274/2280 (99.74%)
 Mismatch: tasks/coverage/babel/packages/babel-parser/test/fixtures/comments/basic/class-method/input.js
 
 Mismatch: tasks/coverage/babel/packages/babel-parser/test/fixtures/comments/basic/class-private-method/input.js
@@ -15,7 +15,3 @@ Mismatch: tasks/coverage/babel/packages/babel-parser/test/fixtures/comments/basi
 
 Mismatch: tasks/coverage/babel/packages/babel-parser/test/fixtures/comments/basic/sequence-expression/input.js
 
-Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/es2022/class-properties/escaped-in-after-initializer/input.js
-Cannot assign to this expression
-Expect to Parse: tasks/coverage/babel/packages/babel-parser/test/fixtures/typescript/class/escaped-in-after-initializer/input.ts
-Cannot assign to this expression