What to test in API testing

 What we can test using API testing?

• Endpoint functionality
• Verify each endpoint performs the intended functionality
• HTTP methods
• Verify each HTTP method performs the intended functionality
• Request and response validation
• Verify that the payload and response follow the correct format of JSON
• Verify that it adheres to the API documentation functionality
• JSON schema validation
• Data validation
• Verify response data is correct
• Verify it gives the correct response for different query params
• Verify it gives the correct response for different path params
• Validate the correctness of the payload
• Error handling
• Verify it gives the correct response codes
• Verify it gives the proper message in the response body for errors
• Verify it gives the correct response code which matches the client, server, and errors
• Verify that it gives the correct error code and response when sending invalid data
• Security testing
• Verify authentication mechanism
• Verify that the authorization mechanism is correct 
• Performance testing
• Verify response time
• Throughput (no of requests per second)
• Error rate
• Mandatory and optional values
• Verify request with mandatory properties in payload and query params
• Verify request with only optional values in payload and query params
• Verify json scheme
• Response and payload should include the correct format of JSON
• Functional testing
• Verify that data is saved correctly in the DB