Number(undefined) returns NaN (Not a Number). When the Number() function is used to convert undefined to a number, the result is NaN because undefined cannot be meaningfully represented as a number. This is different from Number(null), which returns 0. The behavior of NaN in calculations is special: any arithmetic operation involving NaN results in NaN, and NaN is not equal to any value, including itself (NaN !== NaN). The function isNaN() can be used to check if a value is NaN.