Why does Fetch API Send the first PUT request as OPTIONS

Question

I am trying to make a cors PUT request using the vanilla Fetch API

When I click my button to send the PUT request, the method on the first request is OPTIONS. It is only when I click the button again, then the method on the request changes to PUT. Why?

I understand this is part of the CORS preflight, but is there a way to trigger the preflight manually so the OPTIONS response can be cached?

Could this behavior be indicative of a failing promise somewhere?

Answer 1

See the Fetch Standard, section 4.7. CORS-preflight fetch.

Note: This is effectively the user agent implementation of the check to see if the CORS protocol is understood. The so-called CORS-preflight request. If successful it populates the CORS-preflight cache to minimize the number of these fetches.

at steps 1 through 7; also 4.8. CORS-preflight cache.