What should be given special attention when testing APIs?

When testing APIs, it is important to pay attention to the following aspects:

• Correctness of responses: Verify that the API returns the expected data in the correct format.
• Error handling: Ensure that the API returns understandable and correct error codes for incorrect requests.
• Authentication and authorization: Check that access to protected resources is limited and implemented correctly.
• Performance: Evaluate response times and the stability of the API under load.
• Compatibility: Test the API's operation with different client versions and in various environments.
• Security: Test for vulnerabilities such as injections and data leaks.
• Idempotency and state: Verify that methods that should be idempotent (e.g., PUT) are indeed so.

An example of a simple test for checking a GET request using curl:

curl -i https://api.example.com/users/123

This checks that the response contains a status 200 and correct user data.