JavaScript: what is NaN, Object or primitive?

问题

what is NaN, Object or primitive?

NaN - Not a Number

回答1:

NaN is a primitive Number value. Just like 1, 2, etc.

回答2:

It's a primitive. You can check in a number of ways:

• typeof NaN gives "number," not "object."

• Add a property, it disappears. NaN.foo = "hi"; console.log(NaN.foo) // undefined

• NaN instanceof Number gives false (but we know it's a number, so it must be a primitive).

It wouldn't really make sense for NaN to be an object, because expressions like 0 / 0 need to result in NaN, and math operations always result in primitives. Having NaN as an object would also mean it could not act as a falsey value, which it does in some cases.

回答3:

NaN is a property of the global object.

The initial value of NaN is Not-A-Number — the same as the value of Number.NaN. In modern browsers, NaN is a non-configurable, non-writable property. Even when this is not the case, avoid overriding it.

It is rather rare to use NaN in a program. It is the returned value when Math functions fail (Math.sqrt(-1)) or when a function trying to parse a number fails (parseInt("blabla")).

Reference

来源：https://stackoverflow.com/questions/10498511/javascript-what-is-nan-object-or-primitive