This request is only valid within same origin code example

Example 1: access-control-allow-origin

Access-Control-Allow-Origin: *

Example 2: allow cross origin

Access-Control-Allow-Origin: *
Access-Control-Allow-Origin: <origin>
Access-Control-Allow-Origin: null

Example 3: header access-control particular domain

Access-Control-Allow-Origin: http://siteA.com