Moderate HTTP Question 77 of 226

What is a CORS preflight request?

MERN Full Stack · Speak this in 60–90 seconds · Faridabad & Delhi NCR

PICTURE THIS: A REST CALL

ClientGET /users/1

ServerFind row

JSON back200 OK

Simple meaning

For non-simple cross-origin calls, the browser first sends an OPTIONS request asking if the real method, headers, and origin are allowed.

WHY — HTTP instead of guessing?

Why interviewers care about HTTP:

HTTP questions separate people

who only read docs from people who shipped.

Keep it short, concrete,

and tied to Full Stack work.

Stay structured

Name the idea, why it exists, then one short example.

Close cleanly

End with when you use it and one common pitfall.

STEPS — What happens step by step?

Before you speak the answer, walk the interviewer through these steps:

1
For non-simple cross-origin calls,
the browser first sends an OPTIONS request asking if the real method, headers, and origin are allowed.
2
The API must answer
with Access-Control-Allow-* headers.
3
If preflight fails, the
actual POST never leaves the browser.
4
Give an example
One tiny concrete case you can say aloud.
5
Common mistake
What juniors usually get wrong.
6
Close
When you pick this over the alternative.

EXAMPLE — See it in action

Here's a short line you can speak, broken into clear beats:

Say this line

“The API must answer with Access-Control-Allow-* headers.”

Break into beats

TheAPImustanswerwithAccess

Speaking order

2987408337471632900

Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.

Key takeaway

For non-simple cross-origin calls, the browser first sends an OPTIONS request asking if the real method, headers, and origin are allowed. The API must answer with Access-Control-Allow-* headers.