refactor(fetch): use idiomatic downcast for Headers iterator

Author: iammdzaidalam

core/runtime/src/fetch/mod.rs

@@ -213,17 +213,13 @@ fn headers_symbol_iterator(
         || js_error!(TypeError: "`Headers.prototype[Symbol.iterator]` requires a `Headers` object"),
     )?;
 
-    if !this_object.is::<JsHeaders>() {
+    let Ok(headers) = this_object.clone().downcast::<JsHeaders>() else {
         return Err(
             js_error!(TypeError: "`Headers.prototype[Symbol.iterator]` requires a `Headers` object"),
         );
-    }
+    };
 
-    HeadersIterator::create_headers_iterator(
-        this_object.clone().downcast().expect("checked above"),
-        IterationKind::KeyAndValue,
-        context,
-    )
+    HeadersIterator::create_headers_iterator(headers, IterationKind::KeyAndValue, context)
 }
 
 /// Register the `fetch` function in the realm, as well as ALL supporting classes.

core/runtime/src/fetch/tests/headers.rs

@@ -119,3 +119,30 @@ fn headers_invalid_inputs_throw_type_error_objects() {
         ),
     ]);
 }
+
+#[test]
+fn headers_iterator_throws_on_invalid_this() {
+    run_test_actions([
+        TestAction::harness(),
+        TestAction::inspect_context(register),
+        TestAction::run(
+            r#"
+                try {
+                    const iterator = Headers.prototype[Symbol.iterator].call({});
+                    throw Error("expected the call above to throw");
+                } catch (e) {
+                    assert(e instanceof TypeError);
+                    assertEq(e.message, "`Headers.prototype[Symbol.iterator]` requires a `Headers` object");
+                }
+
+                try {
+                    const iterator = Headers.prototype[Symbol.iterator].call(1);
+                    throw Error("expected the call above to throw");
+                } catch (e) {
+                    assert(e instanceof TypeError);
+                    assertEq(e.message, "`Headers.prototype[Symbol.iterator]` requires a `Headers` object");
+                }
+            "#,
+        ),
+    ]);
+}