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| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Preflight request - Glossary | MDN | 1/3 | https://developer.mozilla.org/en-US/docs/Glossary/Preflight_request | reference | web, html, css, javascript, documentation | 2026-05-05T05:40:38.987979+00:00 | kb-cron |
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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:
If the server allows it, then it will respond to the preflight request with an Access-Control-Allow-Methods response header, which lists DELETE:
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.