Is NaN falsy? Why NaN === false returns false

Question

1. Why NaN === false => false, isn\'t NaN falsy?
2. Why NaN === NaN => false, but !!NaN === !!NaN => true

I\'v

Answer 1

1. Why NaN === false => false, isn't NaN falsy?

NaN as you are using, is a global property initialized with value of Not-A-Number. It's not boolean. It's NaN data type as defined by IEEE 754.

It's the "same thing" you compare null === false (or even null == false).

In this case, there is no difference using sctric equal or not: NaN == false, also will return false!

1. Why NaN === NaN => false, but !!NaN === !!NaN => true

2.1. NaN ==== NaN, is false by definition.

2.2. But in !!NaN === !!NaN, you aren't comparing NaNs anymore, when you do ![value], you "evaluate" it (or cast to a boolean).

I'm gonna now explain with null, because it's more used, so you can apply it to NaN:

Casting NaN, it's the same thing that casting null.

null == false? true : false     // false! because it's not a bool false.

!null? true: false              // true! because !null -> true -> if(true)...

More Refs:

http://www.ecma-international.org/ecma-262/5.1/#sec-15.1.1.1
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN