From efd8fbe688e495db90602d7990ef371ad61a13f5 Mon Sep 17 00:00:00 2001
From: Yarchik <spoko.dev@gmail.com>
Date: Wed, 17 Jun 2026 16:06:22 +0100
Subject: [PATCH] fix: keep `__proto__` keys as own properties

`__proto__` as an identifier or string key in an object literal is the
prototype setter, not an own property. So an object with an own
`__proto__` key serialized to `{__proto__:value}`, and `eval` applied it
as the prototype and dropped the entry. Emit `__proto__` as a computed
key (`{['__proto__']:value}`) so it round-trips as an own property.
---
 src/index.spec.ts | 5 +++++
 src/quote.ts      | 3 +++
 2 files changed, 8 insertions(+)

diff --git a/src/index.spec.ts b/src/index.spec.ts
index e03d411..44e77c7 100644
--- a/src/index.spec.ts
+++ b/src/index.spec.ts
@@ -134,6 +134,11 @@ describe("javascript-stringify", () => {
         "should not quote Object.prototype keys",
         test({ constructor: 1, toString: 2 }, "{constructor:1,toString:2}"),
       );
+
+      it(
+        "should keep `__proto__` keys as own properties",
+        testRoundTrip("{['__proto__']:42}"),
+      );
     });
 
     describe("functions", () => {
diff --git a/src/quote.ts b/src/quote.ts
index 225b158..71f955a 100644
--- a/src/quote.ts
+++ b/src/quote.ts
@@ -77,6 +77,9 @@ export function isValidVariableName(name: PropertyKey): name is string {
  * Quote JavaScript key access.
  */
 export function quoteKey(key: PropertyKey, next: Next) {
+  // `__proto__` as an identifier or string key is the prototype setter rather
+  // than an own property; emit it as a computed key so it round-trips.
+  if (key === "__proto__") return `[${next(key)}]`;
   return isValidVariableName(key) ? key : next(key);
 }