Only check if key in obj if obj[key] returns undefined (#3963)

This is a minor optimization in `JsObject_GetString`. We only need to check if `key in obj`
when `obj[key]` returns `undefined` to distinguish between missing and present but set
to `undefined`.

src/core/hiwire.c

@@ -966,12 +966,15 @@ EM_JS(int, normalizeReservedWords, (int word), {
 });
 
 EM_JS_REF(JsRef, JsObject_GetString, (JsRef idobj, const char* ptrkey), {
-  let jsobj = Hiwire.get_value(idobj);
-  let jskey = normalizeReservedWords(UTF8ToString(ptrkey));
-  if (jskey in jsobj) {
-    return Hiwire.new_value(jsobj[jskey]);
+  const jsobj = Hiwire.get_value(idobj);
+  const jskey = normalizeReservedWords(UTF8ToString(ptrkey));
+  const result = jsobj[jskey];
+  // clang-format off
+  if (result === undefined && !(jskey in jsobj)) {
+    // clang-format on
+    return ERROR_REF;
   }
-  return ERROR_REF;
+  return Hiwire.new_value(result);
 });
 
 // clang-format off

src/tests/test_jsproxy.py

@@ -2431,3 +2431,32 @@ def test_revoked_proxy(selenium):
 
     x = run_js("(p = Proxy.revocable({}, {})); p.revoke(); p.proxy")
     run_js("((x) => x)")(x)
+
+
+@run_in_pyodide
+def test_js_proxy_attribute(selenium):
+    # Check that `in` is only consulted as a fallback if indexing returns None
+    import pytest
+
+    from pyodide.code import run_js
+
+    x = run_js(
+        """
+        new Proxy(
+            {},
+            {
+                get(target, val) {
+                    return { a: 3, b: 7, c: undefined, d: undefined }[val];
+                },
+                has(target, val) {
+                    return { a: true, b: false, c: true, d: false }[val];
+                },
+            }
+        );
+        """
+    )
+    assert x.a == 3
+    assert x.b == 7  # Previously this raised AttributeError
+    assert x.c is None
+    with pytest.raises(AttributeError):
+        x.d