What HTTP status code should you return for invalid input?

Question

Accepted Answer

Return 400 Bad Request whenever the client sends invalid or malformed input.
if (!isValidInput(req.query)) {
    res.status(400);
    res.type('application/json');
    res.send(JSON.stringify({ error: 'Invalid input' }));
    return;
}

Common status codes:

Code
Meaning
Use Case

200
OK
Successful request

201
Created
Resource created (POST)

400
Bad Request
Invalid input

401
Unauthorized
Not authenticated

403
Forbidden
Not authorized

404
Not Found
Resource doesn't exist

500
Server Error
Unexpected server problem

Tip: Always return appropriate status codes - clients depend on them to handle responses correctly.

Code	Meaning	Use Case
200	OK	Successful request
201	Created	Resource created (POST)
400	Bad Request	Invalid input
401	Unauthorized	Not authenticated
403	Forbidden	Not authorized
404	Not Found	Resource doesn't exist
500	Server Error	Unexpected server problem