How to test the functionality 1 time and be sure that it is not “broken”?

Question

Programming

In manual testing (example: a web page), it is very common to re-test the same thing when I add new functionality. Example: There was a web page with five buttons, each of which changes the state of an object. When I add the sixth button, I double check all the previous five buttons to be sure. Since the new button introduces new code to change the state of the object, and if something went wrong, the previous buttons may stop working.

Answer 1

No way. regression testing is just doing what checks if nothing from the old functionality has broken.
Automate.

Answer 2

in addition to Saboteur 's answer 's answer :
google "TDD", "unit testing")

Answer 3

make buttons independent from each other and bind only the functionality that is relevant to them, instead of hanging everything in a row
, let's say all a button can do is receive an event and pass it on to the others - what could be errors if you add a dozen more of the same buttons?