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=== means value and type are equal. In this case, the types are equal, but the values are not. I'd argue that NaN === NaN being true would imply a broken implementation of IEEE 754, and the designers of JS seem to agree with me.

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What benefit is created by having a hierarchy of comparison operators in which there is no actual way to compare two numbers for semantic (not numeric) equivalence?

If (NaN == NaN) == false, and (NaN === NaN) == false, how would you implement an isNaN() function? If your answer is 'depend on the VM', I find that a bit strange.

typeof(NaN) should still be 'number', which would identify it uniquely. The irony of the value "Not a Number" having type "Number" is not lost on me, however.

function isNaN(x) { return x != x; };

new String("foo") === new String("foo") is false. === does not mean value and type are equal, it means that the left is the same object as on the right OR that both the left and right are primitives with the same value.

The behavior there is a result of the 'new' operator. typeof new String("foo") is specified to be "object", and it is extensible (String("foo") is not). Both == and === on two object-types return true if both refer to the same thing. More generally, when two types are the same, == has the same behavior as ===.

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