What does changing an object by reference mean, and why do we copy state?

When I assign one object to two variables, both can point to the same object. A change made through one variable is then visible through the other. That behaves differently from copying a plain number.

In React, instead of changing an existing state object, I usually create a new one, e.g. {...user, name: 'Ola'}. That makes the change explicit and I don't modify data used by earlier renders. The spread operator makes a shallow copy: a nested object can still be shared. If I change a nested field, I copy that level too. But there's no need to blindly deep-copy everything.