improve stringwrapper bytestring unwrap

src/workerd/jsg/value.h

@@ -468,18 +468,19 @@ class StringWrapper {
       kj::Maybe<v8::Local<v8::Object>> parentObject) {
     // TODO(cleanup): Move to a HeaderStringWrapper in the api directory.
     v8::Local<v8::String> str = check(handle->ToString(context));
-    auto result =
-        ByteString(KJ_ASSERT_NONNULL(tryUnwrap(context, str, (kj::String*)nullptr, parentObject)));
-    if (str->ContainsOnlyOneByte()) {
-      auto buf = kj::heapArray<kj::byte>(str->Length() + 1);
-      str->WriteOneByte(context->GetIsolate(), buf.begin());
-      for (auto b: buf) {
-        if (b >= 128) {
-          result.warning = ByteString::Warning::CONTAINS_EXTENDED_ASCII;
-          break;
-        }
-      }
+    auto result = ByteString(KJ_ASSERT_NONNULL(
+        tryUnwrap(context, str, static_cast<kj::String*>(nullptr), parentObject)));
+
+    v8::String::ValueView str_view(context->GetIsolate(), str);
+
+    if (str_view.is_one_byte()) [[likely]] {
+      // Storage is one-byte. We don't need to scan.
+    } else if (str->ContainsOnlyOneByte()) {
+      // Storage is two-byte, but it contains only one byte.
+      // No need to scan the string for extended ascii.
+      result.warning = ByteString::Warning::CONTAINS_EXTENDED_ASCII;
     } else {
+      // Storage is two-bytes, and it doesn't contain "only one-bytes"
       result.warning = ByteString::Warning::CONTAINS_UNICODE;
     }
     return kj::mv(result);