So I have been using CORS for a short time now, I just read up on how it works behind the scenes.

My question is how does the browser know when to do a pre-flight request?

There is no issue involved right now, as the browser seems to be working fine, I am just a little bit curious. Because when I make the request, I am not giving it any information on whether or not it is CORS, yet it still knows to do a pre-flight.

So I have been using CORS for a short time now, I just read up on how it works behind the scenes.

My question is how does the browser know when to do a pre-flight request?

There is no issue involved right now, as the browser seems to be working fine, I am just a little bit curious. Because when I make the request, I am not giving it any information on whether or not it is CORS, yet it still knows to do a pre-flight.

• javascript
• rest
• dom
• xmlhttprequest
• cors

Share Improve this question Follow asked Jun 17, 2015 at 17:12 Qasim AhmedQasim Ahmed 19722 silver badges1010 bronze badges

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Sorted by: Reset to default 7

The browser will send a preflight request if:

• You add custom headers to your request
• You use a method other than GET, HEAD or POST
• You use POST with an unusual Content-Type.

More details here: HTTP access control (CORS), Preflighted requests