From c41be0bf9c367c12394a0436a5699ffef8acfc50 Mon Sep 17 00:00:00 2001
From: Cole Ellison <cellison@figma.com>
Date: Sat, 12 Sep 2026 15:06:14 -0400
Subject: [PATCH] Correct identification of `in`/`instanceof` identifiers
 (#20019)

Co-authored-by: fisker Cheung <lionkay@gmail.com>
---
 changelog_unreleased/javascript/20019.md      | 22 +++++++++++++++
 src/language-js/print/class-body.js           |  4 +--
 .../no-semi/__snapshots__/format.test.js.snap | 28 +++++++++++++++++++
 tests/format/js/no-semi/private-field.js      |  7 +++++
 4 files changed, 59 insertions(+), 2 deletions(-)
 create mode 100644 changelog_unreleased/javascript/20019.md

diff --git a/changelog_unreleased/javascript/20019.md b/changelog_unreleased/javascript/20019.md
new file mode 100644
index 000000000..2e72e2a77
--- /dev/null
+++ b/changelog_unreleased/javascript/20019.md
@@ -0,0 +1,22 @@
+#### Avoid unnecessary semicolons before private members named `#in` or `#instanceof` (#20019 by @cellison-figma)
+
+<!-- prettier-ignore -->
+```js
+// Input
+class Graph {
+  #nodes = {};
+  #in = {};
+}
+
+// Prettier stable (--parser oxc --no-semi)
+class Graph {
+  #nodes = {};
+  #in = {}
+}
+
+// Prettier main (--parser oxc --no-semi)
+class Graph {
+  #nodes = {}
+  #in = {}
+}
+```
diff --git a/src/language-js/print/class-body.js b/src/language-js/print/class-body.js
index 56fb7d258..2df22f50b 100644
--- a/src/language-js/print/class-body.js
+++ b/src/language-js/print/class-body.js
@@ -256,8 +256,8 @@ function shouldPrintSemicolonAfterClassProperty(
     return false;
   }
 
-  if (!nextNode.computed) {
-    const name = nextNode.key?.name;
+  if (!nextNode.computed && nextNode.key?.type === "Identifier") {
+    const { name } = nextNode.key;
     if (name === "in" || name === "instanceof") {
       return true;
     }
diff --git a/tests/format/js/no-semi/__snapshots__/format.test.js.snap b/tests/format/js/no-semi/__snapshots__/format.test.js.snap
index cbc91e3b8..46e81a55e 100644
--- a/tests/format/js/no-semi/__snapshots__/format.test.js.snap
+++ b/tests/format/js/no-semi/__snapshots__/format.test.js.snap
@@ -2104,12 +2104,26 @@ class C {
   ["method"]() {}
 }
 
+class PrivateKeywordFields {
+  a = {};
+  #in = {};
+  b = {};
+  #instanceof = {};
+}
+
 =====================================output=====================================
 class C {
   #field = "value";
   ["method"]() {}
 }
 
+class PrivateKeywordFields {
+  a = {}
+  #in = {}
+  b = {}
+  #instanceof = {}
+}
+
 ================================================================================
 `;
 
@@ -2123,12 +2137,26 @@ class C {
   ["method"]() {}
 }
 
+class PrivateKeywordFields {
+  a = {};
+  #in = {};
+  b = {};
+  #instanceof = {};
+}
+
 =====================================output=====================================
 class C {
   #field = "value";
   ["method"]() {}
 }
 
+class PrivateKeywordFields {
+  a = {};
+  #in = {};
+  b = {};
+  #instanceof = {};
+}
+
 ================================================================================
 `;
 
diff --git a/tests/format/js/no-semi/private-field.js b/tests/format/js/no-semi/private-field.js
index feca225a4..47fab1340 100644
--- a/tests/format/js/no-semi/private-field.js
+++ b/tests/format/js/no-semi/private-field.js
@@ -2,3 +2,10 @@ class C {
   #field = 'value';
   ["method"]() {}
 }
+
+class PrivateKeywordFields {
+  a = {};
+  #in = {};
+  b = {};
+  #instanceof = {};
+}