diff --git a/Userland/Libraries/LibJS/Runtime/PrimitiveString.h b/Userland/Libraries/LibJS/Runtime/PrimitiveString.h
index 5f27e63b50..dd15cb4d49 100644
--- a/Userland/Libraries/LibJS/Runtime/PrimitiveString.h
+++ b/Userland/Libraries/LibJS/Runtime/PrimitiveString.h
@@ -22,9 +22,11 @@ public:
     PrimitiveString& operator=(PrimitiveString const&) = delete;
 
     String const& string() const;
+    bool has_utf8_string() const { return m_has_utf8_string; }
 
     Utf16String const& utf16_string() const;
     Utf16View utf16_string_view() const;
+    bool has_utf16_string() const { return m_has_utf16_string; }
 
 private:
     virtual const char* class_name() const override { return "PrimitiveString"; }
diff --git a/Userland/Libraries/LibJS/Runtime/Value.cpp b/Userland/Libraries/LibJS/Runtime/Value.cpp
index db3e22cb9c..eb8416fadd 100644
--- a/Userland/Libraries/LibJS/Runtime/Value.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Value.cpp
@@ -1100,6 +1100,21 @@ Value add(GlobalObject& global_object, Value lhs, Value rhs)
     if (vm.exception())
         return {};
 
+    if (lhs_primitive.is_string() && rhs_primitive.is_string()) {
+        auto const& lhs_string = lhs_primitive.as_string();
+        auto const& rhs_string = rhs_primitive.as_string();
+
+        if (lhs_string.has_utf16_string() && rhs_string.has_utf16_string()) {
+            auto const& lhs_utf16_string = lhs_string.utf16_string();
+            auto const& rhs_utf16_string = rhs_string.utf16_string();
+
+            Vector<u16> combined;
+            combined.ensure_capacity(lhs_utf16_string.length_in_code_units() + rhs_utf16_string.length_in_code_units());
+            combined.extend(lhs_utf16_string.string());
+            combined.extend(rhs_utf16_string.string());
+            return js_string(vm.heap(), Utf16String(move(combined)));
+        }
+    }
     if (lhs_primitive.is_string() || rhs_primitive.is_string()) {
         auto lhs_string = lhs_primitive.to_string(global_object);
         if (vm.exception())