Preflight request

A CORS preflight request is a CORS request that checks to see if the CORS protocol is understood and a server is aware using specific methods and headers.

It is an OPTIONS request, using two or three HTTP request headers: Access-Control-Request-Method, Origin, and optionally Access-Control-Request-Headers.

A preflight request is automatically issued by a browser and in normal cases, front-end developers don't need to craft such requests themselves. It appears when request is qualified as "to be preflighted" and omitted for simple requests.

For example, a client might be asking a server if it would allow a DELETE request, before sending a DELETE request, by using a preflight request:

OPTIONS /resource/foo
Access-Control-Request-Method: DELETE
Access-Control-Request-Headers: x-requested-with
Origin: https://foo.bar.org

If the server allows it, then it will respond to the preflight request with an Access-Control-Allow-Methods response header, which lists DELETE:

HTTP/1.1 204 No Content
Connection: keep-alive
Access-Control-Allow-Origin: https://foo.bar.org
Access-Control-Allow-Methods: POST, GET, OPTIONS, DELETE
Access-Control-Allow-Headers: X-Requested-With
Access-Control-Max-Age: 86400

The preflight response can be optionally cached for the requests created in the same URL using Access-Control-Max-Age header like in the above example. To cache preflight responses, the browser uses a specific cache that is separate from the general HTTP cache that the browser manages. Preflight responses are never cached in the browser's general HTTP cache.