Why 'NaN' and 'Undefined' are not reserved keywords in JavaScript?

I cannot tell you why, but undefined and NaN are actually properties of the global object:

15.1.1 Value Properties of the Global Object

15.1.1.1 NaN
The value of NaN is NaN (see 8.5). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.
(...)
15.1.1.3 undefined
The value of undefined is undefined (see 8.1). This property has the attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }.

There is a difference between the value undefined (NaN) and the corresponding property.

You might notice the [[Writable]]: false. I'm not sure whether this new in ES5 (and might not be adapted by all browsers), but in newer browsers (tested in Firefox 6), assigning a new value to undefined has no effect:

[12:28:15.090] < undefined = "foo"
[12:28:15.093] > "foo"
[12:28:19.882] < undefined
[12:28:19.883] > undefined

So although it seems you can assign a new value, you actually cannot.

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Why they are not reserved keywords?

Not sure if there was a specific reason to not make them reserved keywords, but it was decided against it. The language still works. You cannot override these values, so it's fine.

The only way to test whether a number is NaN, is to use isNaN() anyway.

I'm speculating now, but the reason why I think NaN and undefined are not keywords is because you generally don't assign these values to variables.

var x = undefined; // doesn't make sense, use null!
var y = NaN; // You can't do much with this variable!

undefined basically means uninitialized, and if you want to make it clear that the value is unknown you use null. So undefined usually means not-initialized or the result of JavaScript code gone wrong.

NaN Is rarely assigned manually, simply because you can't do much with this value. It is usually the result of a calculation gone wrong. The makers of JavaScript probably didn't want to give this value the visibility of primitive values.

Also, NaN is also present in other languages and it isn't used as a keyword there either. For example: In C# NaN is represented by Double.NaN, since you don't make a distinction between floating point and integer values in JavaScript, I'm guessing that's why they put NaN with the Global Identifiers!

I hope this clears things up!

https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/NaN

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.

https://developer.mozilla.org/en/JavaScript/Reference/Global_Objects/undefined

undefined is a property of the global object, i.e. it is a variable in global scope.

The initial value of undefined is the primitive value undefined.