So, rather return the whole object first, just wrap it to json_encode and then return it. This will return a proper and valid object.
public function id($id){
$promotion = Promotion::find($id);
return json_encode($promotion);
}
Or, For DB this will be just like,
public function id($id){
$promotion = DB::table('promotions')->first();
return json_encode($promotion);
}
I think it may help someone else.