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.
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: origin, 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
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-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.