I need to know if it's possible to do functional testing (frontend or backend) with JMeter only - w/o 3rd party software. I need to be able to test required fields and bad data input in a form.
You cannot test frontend using JMeter mainly due to its main constraint, as per JMeter project main page:
However you can easily send invalid data inside a HTTP Request and then use Response Assertion to ensure that the response contains expected value(s) or vice versa, doesn't contain something which it shouldn't be containing. You can also mark HTTP status codes above 400 as successful by ticking
Ignore Status box
After reviewing the HTTP status codes, I couldn't find anything that would relate to invalid data. For example, if there is a required field that a user does not fill out in a form and hits save, what status code would be used?
Response Assertions I could use are Request Headers and Response Data (for drop down fields). Any other assertion you would suggest for invalid data? What could I use for fields that a user would type in like, Organization Name or First Name, Last Name?