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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. Preflight requests are 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 ommited for simple requests. The preflight response can be optionally cached for the requests created in the same URL using Access-Control-Max-Age header.