References: Two Names, One Object

Why copying a variable that holds an object copies only the note that says where it is, so two names can change one object; how to make a real copy; and null, None and undefined, the note that points at nothing.

Updated

Cheat sheet: References: Two Names, One Object
Category
Data Structures
Level
Apprentice
Tags
c-sharp, java, JavaScript, Null (software-development), Object (oop), python, Reference (software-development), typescript