What is CORS and why does a MERN app hit it in development?
PICTURE THIS: STACK VS QUEUE
Simple meaning
CORS is a browser rule that blocks a page on one origin from reading another origin unless the server allows it.
WHY — Security instead of guessing?
Why interviewers care about Security:
who only read docs from people who shipped.
and tied to Full Stack work.
Name the idea, why it exists, then one short example.
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:
- 1CORS is a browser
rule that blocks a page on one origin from reading another origin unless the server allows it.
- 2A React app on
one port and Express on another are different origins.
- 3Configure cors middleware with
explicit allowed origins instead of a wildcard in production.
- 4Give an example
One tiny concrete case you can say aloud.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
CORS is a browser rule that blocks a page on one origin from reading another origin unless the server allows it. A React app on one port and Express on another are different origins.