From fb9084a36124b6b2e302f9e9777a5e38d716ecef Mon Sep 17 00:00:00 2001
From: "Chris (ChrisJr404)" <11917633+ChrisJr404@users.noreply.github.com>
Date: Mon, 24 Aug 2026 21:01:36 -0400
Subject: [PATCH] feat(engine): include the value type in "not a constructor"
 errors

---
 core/engine/src/builtins/function/tests.rs    |  2 +-
 .../engine/src/object/internal_methods/mod.rs | 11 +++++--
 core/engine/src/tests/function.rs             | 29 ++++++++++++++++++-
 core/engine/src/vm/opcode/new/mod.rs          | 10 +++++--
 4 files changed, 46 insertions(+), 6 deletions(-)

diff --git a/core/engine/src/builtins/function/tests.rs b/core/engine/src/builtins/function/tests.rs
index 7200a610e3c..e33a7526da0 100644
--- a/core/engine/src/builtins/function/tests.rs
+++ b/core/engine/src/builtins/function/tests.rs
@@ -62,7 +62,7 @@ fn function_prototype() {
         TestAction::assert_native_error(
             "new Function.prototype()",
             JsNativeErrorKind::Type,
-            "not a constructor",
+            "function is not a constructor",
         ),
     ]);
 }
diff --git a/core/engine/src/object/internal_methods/mod.rs b/core/engine/src/object/internal_methods/mod.rs
index 8e474b5dde5..f09998af056 100644
--- a/core/engine/src/object/internal_methods/mod.rs
+++ b/core/engine/src/object/internal_methods/mod.rs
@@ -1207,12 +1207,19 @@ fn non_existent_call(
 }
 
 fn non_existent_construct(
-    _obj: &JsObject,
+    obj: &JsObject,
     _argument_count: usize,
     context: &mut InternalMethodCallContext<'_>,
 ) -> JsResult<CallValue> {
+    // A callable object reaching this point is a function without a [[Construct]]
+    // slot (e.g. an arrow function or method), so report it as a function.
+    let type_of = if obj.is_callable() {
+        "function"
+    } else {
+        "object"
+    };
     Err(JsNativeError::typ()
-        .with_message("not a constructor")
+        .with_message(format!("{type_of} is not a constructor"))
         .with_realm(context.realm().clone())
         .into())
 }
diff --git a/core/engine/src/tests/function.rs b/core/engine/src/tests/function.rs
index 84cb0c8b777..1c7f2be453c 100644
--- a/core/engine/src/tests/function.rs
+++ b/core/engine/src/tests/function.rs
@@ -215,10 +215,37 @@ fn should_type_error_when_new_is_not_constructor() {
     run_test_actions([TestAction::assert_native_error(
         "new ''()",
         JsNativeErrorKind::Type,
-        "not a constructor",
+        "string is not a constructor",
     )]);
 }
 
+#[test]
+fn new_on_non_constructor_reports_the_value_type() {
+    run_test_actions([
+        TestAction::assert_native_error(
+            "new 5()",
+            JsNativeErrorKind::Type,
+            "number is not a constructor",
+        ),
+        TestAction::assert_native_error(
+            "new undefined()",
+            JsNativeErrorKind::Type,
+            "undefined is not a constructor",
+        ),
+        TestAction::assert_native_error(
+            "new true()",
+            JsNativeErrorKind::Type,
+            "boolean is not a constructor",
+        ),
+        // A callable object without a [[Construct]] slot, such as an arrow function.
+        TestAction::assert_native_error(
+            "new (() => {})()",
+            JsNativeErrorKind::Type,
+            "function is not a constructor",
+        ),
+    ]);
+}
+
 #[test]
 fn new_instance_should_point_to_prototype() {
     // A new instance should point to a prototype object created with the constructor function
diff --git a/core/engine/src/vm/opcode/new/mod.rs b/core/engine/src/vm/opcode/new/mod.rs
index d54c7e2bdeb..c4bd5d343b5 100644
--- a/core/engine/src/vm/opcode/new/mod.rs
+++ b/core/engine/src/vm/opcode/new/mod.rs
@@ -18,7 +18,10 @@ impl New {
 
         let cons = func
             .as_object()
-            .ok_or_else(|| JsNativeError::typ().with_message("not a constructor"))?
+            .ok_or_else(|| {
+                JsNativeError::typ()
+                    .with_message(format!("{} is not a constructor", func.type_of()))
+            })?
             .clone();
 
         context.vm.stack.push(cons.clone()); // Push new.target
@@ -59,7 +62,10 @@ impl NewSpread {
 
         let cons = func
             .as_object()
-            .ok_or_else(|| JsNativeError::typ().with_message("not a constructor"))?
+            .ok_or_else(|| {
+                JsNativeError::typ()
+                    .with_message(format!("{} is not a constructor", func.type_of()))
+            })?
             .clone();
 
         let argument_count = arguments.len();