A preflight request is automatically issued by a browser, when needed. In normal cases, front-end developers don't need to craft such requests themselves.
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