From 4becd1167a31105dedf37b21653600e5ff290d39 Mon Sep 17 00:00:00 2001
From: robobun <117481402+robobun@users.noreply.github.com>
Date: Mon, 9 Nov 2026 00:56:16 +0000
Subject: [PATCH] [JSC] The inspector shows `boundThis` of an arrow function as
 undefined while `this` of its derived constructor has no value

JSInjectedScriptHost::getInternalProperties() reads `this` of an arrow function from the scope of the function,
for the internal property `boundThis`. In a derived constructor that scope slot holds the empty value until
super() returns. constructInternalProperty() stored the empty value with putDirect(), and the process aborted.

A debugger client sends the request when the user expands the arrow function in its variables view, while the
program is paused in the constructor before super().

The empty value is undefined now, as for a variable without a value in DebuggerScope::getOwnPropertySlot() and
for `this` of a frame in DebuggerCallFrame::thisValue().
---
 Source/JavaScriptCore/inspector/JSInjectedScriptHost.cpp | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/Source/JavaScriptCore/inspector/JSInjectedScriptHost.cpp b/Source/JavaScriptCore/inspector/JSInjectedScriptHost.cpp
index d18e0c93f8bfb..1b53e0fd4cea3 100644
--- a/Source/JavaScriptCore/inspector/JSInjectedScriptHost.cpp
+++ b/Source/JavaScriptCore/inspector/JSInjectedScriptHost.cpp
@@ -582,6 +582,9 @@ JSValue JSInjectedScriptHost::getInternalProperties(JSGlobalObject* globalObject
             RETURN_IF_EXCEPTION(scope, JSValue());
             JSValue thisObject = jsScope->get(globalObject, vm.propertyNames->builtinNames().thisPrivateName());
             RETURN_IF_EXCEPTION(scope, JSValue());
+            // |this| of a derived constructor has no value before super() returns. As DebuggerScope does for a variable without a value, say undefined.
+            if (!thisObject)
+                thisObject = jsUndefined();
             array->putDirectIndex(globalObject, index++, constructInternalProperty(globalObject, "boundThis"_s, thisObject));
             RETURN_IF_EXCEPTION(scope, JSValue());
             return array;