问题
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(undefined); // false
isFinite(null); // true
I do not understand as I would expect null and undefined to be handled in the same manner.
回答1:
This is because Number(null) === 0.
http://es5.github.io/#x9.3
回答2:
isFinite (number)
Returns false if the argument coerces to NaN, +∞, or −∞, and otherwise returns true.
isFinite convert input using Number() and:
Number(undefined); //== NaN
Number(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.
来源：https://stackoverflow.com/questions/19254205/why-is-isfiniteundefined-isfinitenull