JavaScript NaN equvialence

Question

Just for fun I was trying to compare a value which I set to NaN to another value which I also set to NaN.

Why are two variables, with identical values, not equivalent?

var a = Number.Nan
var b = Number.Nan
a === b
false

a == b
false

var a = NaN
var b = NaN
a === b 
false
a == b
false

Why does this happen?

Answer 1

As @TechnicalChaos said, but here are links to MDN over W3Schools ;)

NaN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/NaN

IsNaN: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/isNaN

The pertinent parts being:

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")).

And:

Unlike all other possible values in JavaScript, it is not possible to rely on the equality operators (== and ===) to determine whether a value is NaN or not, because both NaN == NaN and NaN === NaN evaluate to false. Hence, the necessity of an isNaN function.

Don't think of NaN along the same lines as null or undefined, it's a "special case" :P

But if you're willing to risk it for a biscuit:

NaN.toString() === NaN.toString()
true

But don't do that!

Answer 2

NaN is Not a Number, there is no evaluated numberic value to compare - Use IsNaN() To compare NaN values. W3 Schools link

Answer 3

Keep in mind that a lot of JavaScript isn't great. It was designed in 10 days and a lot (not all) of it may not be intuitive. This is one of those times where you don't think about why and you just use isNaN(someVar) to check for it.