CORS setup with ReactJS Fetch API
I would just like to answer my own question. After reading more documentation about how CORS works, I found out that, to allow Drupal access other site/server, CORS must be enabled in the other party not in Drupal site itself. Some basic explanation found here too. https://stackoverflow.com/questions/10636611/how-does-access-control-allow-origin-header-work/10636765#10636765