Access-Control-Allow-*returned when accessing a domain's URL which has a CORS policy in place.
Access-Control-Allow-Headersheader. If you're unsure about what allowed headers the endpoint supports, look at the preflight Options response. The values displayed on the Access-Control-Allow-Headers determines what can be used.
Access-Control-Allow-Originvalue does not match the domain you are on, then it will fail. If this is the case you will need to communicate with the back-end server owner (domain-B) to determine a solution. For example: they would need to allow the domain (domain-A) you are requesting from.
Access-Control-Allow-Originhas a special value which can be set, an asterisk
*. This is known as a wildcard which means that the back-end server (domain-B) allows requests from any origin (domain-A). Public APIs need this value set in order to allow requests from any origin.
Access-Control-Allow-*headers there are also
Access-Control-Request-*headers. It's important to have a cursory understanding of the available values here to more fully understand how CORS requests are formulated. If you need more information, please refer to the CORS article linked at the top of this guide.
Access-Control-Allow-*were returned from our back-end server. These instructions are the instructions that the browser then follows.
Access-Control-Allow-Originvalue then we will not be able to make the CORS request.
FetchCORS request one setting you will want to be familiar with is
mode. This is the setting which explicitly states what type of request you want to make. It is useful to define this to ensure consistency in how a browser makes requests along with communicating to yourself or other, future developers what the intent and expectations of network requests are.