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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?

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  • Have you read the specification? Commented Feb 18, 2017 at 4:35
  • Yes, I've read the spec. I'm wondering how this is handled -- do I need to check the already sent request to see if it's an OPTIONS request then send it again? Commented Feb 19, 2017 at 23:28
  • Is the specification not clear? Commented Feb 19, 2017 at 23:29
  • No, it's not. Does the preflight fetch happen manually or automatically? Commented Feb 19, 2017 at 23:46
  • Can you include the javascript that you are trying at Question? You could perform the request manually, if, from what can gather, the preflight request is occurring automatically. fetch("url", {method:"OPTIONS"}) .then(response => response.headers) .then(_headers => Array.from(_headers.entries(), h => console.log(h))).catch(err => console.error(err)), else, from reading of specification, a network error would occur. If from your interpretation the specification is not clear, would suggest searching for a similar issue, or filing an issue at github.com/whatwg/fetch/issues Commented Feb 20, 2017 at 0:32

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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.

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