If you declare a state variable but never assign it, what value does it hold?

Its zero value — Solidity has no "null/undefined"; every type initialises to a defined default (0 for numbers, false for bool, the zero address for address, "" for string).

uint256 favoriteNumber;   // this is 0, not undefined

Solidity variables are never uninitialised in the way they might be in other languages. Declaring uint256 favoriteNumber; is identical to uint256 favoriteNumber = 0;. Defaults by type:

This matters for mappings especially: every key of a mapping already "exists" holding the zero value, so a lookup on a never-set key returns 0 rather than an error.

Go deeper:

• Solidity docs: types — each type's default (zero) value.