Fix observable behavior of Intl.DateTimeFormat constructor

Author: Vellumic

core/engine/src/builtins/intl/date_time_format/mod.rs

@@ -177,18 +177,18 @@ impl BuiltInConstructor for DateTimeFormat {
         let options = args.get_or_undefined(1);
 
         // 2. Let dateTimeFormat be ? CreateDateTimeFormat(newTarget, locales, options, any, date).
+        let prototype = get_prototype_from_constructor(
+            new_target_inner,
+            StandardConstructors::date_time_format,
+            context,
+        )?;
         let dtf = create_date_time_format(
             locales,
             options,
             FormatType::Any,
             FormatDefaults::Date,
             context,
         )?;
-        let prototype = get_prototype_from_constructor(
-            new_target_inner,
-            StandardConstructors::date_time_format,
-            context,
-        )?;
         let date_time_format = JsObject::from_proto_and_data(prototype, dtf);
 
         // 3. If the implementation supports the normative optional constructor mode of 4.3 Note 1, then
@@ -534,6 +534,11 @@ pub(crate) fn create_date_time_format(
     defaults: FormatDefaults,
     context: &mut Context,
 ) -> JsResult<DateTimeFormat> {
+    // NOTE: The below step's code was moved out into constructor to prevent unnecessary JsObject allocation when we create dtf internally
+    // (e.g. toLocaleString methods of Date and Temporal objects)
+    // 1. Let dateTimeFormat be ? OrdinaryCreateFromConstructor(newTarget, "%Intl.DateTimeFormat.prototype%",
+    // « [[InitializedDateTimeFormat]], [[Locale]], [[Calendar]], [[NumberingSystem]], [[TimeZone]],
+    // [[HourCycle]], [[DateStyle]], [[TimeStyle]], [[DateTimeFormat]], [[BoundFormat]] »).
     // 2. Let hour12 be undefined. <- TODO
     // 3. Let modifyResolutionOptions be a new Abstract Closure with parameters (options) that captures hour12 and performs the following steps when called:
     //        a. Set hour12 to options.[[hour12]].

core/engine/src/builtins/intl/date_time_format/tests.rs

@@ -88,3 +88,31 @@ fn date_to_locale_string() {
         TestAction::assert_eq("result === '6:07\u{202f}AM'", true),
     ]);
 }
+
+#[cfg(feature = "intl_bundled")]
+#[test]
+fn dtf_ctor_observable_behavior() {
+    run_test_actions([
+        TestAction::run(indoc! {"
+            const expected = [];
+
+            const proxyConstructor = new Proxy(Intl.DateTimeFormat, {
+              get(target, prop) {
+                if (prop === 'prototype') {
+                  expected.push('prototype-access');
+                }
+                return target[prop];
+              }
+            });
+
+            try {
+              new proxyConstructor('en', { timeZone: 'Invalid/Zone' });
+            } catch (e) {
+              expected.push('error-thrown');
+            }
+        "}),
+        TestAction::assert_eq("expected.length === 2", true),
+        TestAction::assert_eq("expected[0] === 'prototype-access'", true),
+        TestAction::assert_eq("expected[1] === 'error-thrown'", true),
+    ]);
+}