Why Is Isfinite(undefined) != Isfinite(null)?

Why is the value for undefined considered Finite in javascript while null is not? This is a very basic question, which has thwarted my googlefoo (too much noise). isFinite(undefine

Solution 1:

This is because Number(null) === 0.

http://es5.github.io/#x9.3

Solution 2:

isFinite (number)

Returns false if the argument coerces to NaN, +∞, or −∞, and otherwise returns true.

isFinite convert input using Number() and:

Number(undefined); //== NaNNumber(null); //== 0

that is the reason undefined is false and null is true for isFinite.

If you try:

isFinite(!undefined); // true

because undefined is NaN and on negating it it converted to 1 which is finite.