What is a CORS error, why does it arise, and how do you fix it?
Only 30% of the candidates could answer it properly.
Let’s get it straight so you never blank out in an interview. 👇
1/ What is CORS?
CORS stands for Cross-Origin Resource Sharing—a security feature built into browsers.
It blocks requests made from one origin (domain, protocol, or port) to another origin unless explicitly allowed by the server.
For example:
- Your frontend is hosted at `frontend.com`.
- Your backend API is hosted at `api.backend.com`.
The browser treats these as different origins and blocks the request unless it’s explicitly allowed.
2/ Why Does It Happen?
CORS errors are triggered by the Same-Origin Policy, which prevents malicious websites from making unauthorized API calls using your credentials.
When the backend server doesn’t include the right CORS headers, the browser refuses to share the response and throws this error:
> *Access to fetch at 'https://api.backend.com' from origin 'https://frontend.com' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present.*
In short, the browser isn’t blocking the request—it’s blocking the response for security reasons.
3/ The Real Problem Behind CORS
- It’s not a frontend issue.
- It’s not a browser bug.
- It’s a server-side configuration issue.
Most candidates panic and assume the browser is the problem. It’s not.
CORS is the server’s responsibility to allow or deny requests from other origins.
4/ How Do You Fix It?
Step 1: Update the Backend.
The server must send the right headers, like:
- `Access-Control-Allow-Origin: *` (Allows all origins).
- Or specify trusted domains like `Access-Control-Allow-Origin: https://frontend.com`.
Step 2: Handle Preflight Requests (OPTIONS).
For complex requests (like `POST` with custom headers), browsers send a preflight request before the actual call.
The server must respond to this with:
- `Access-Control-Allow-Methods: GET, POST, OPTIONS`
- `Access-Control-Allow-Headers: Content-Type, Authorization`
Step 3: Use a Proxy for Local Development.
If the backend isn’t updated yet, set up a proxy to forward requests through the same origin as your frontend.
5/ Can We Bypass CORS?
Short answer—No.
Any hacky workaround, like disabling CORS in the browser or using extensions, won’t work in production.
Fix it properly by configuring the server. That’s the only scalable solution.
6/ Why Is This Important in Interviews?
Because CORS tests whether you:
- Understand how the web works.
- Can debug issues logically.
- Know where the real problem lies (backend vs frontend).
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