First, let me point out that NaN is a very special value: By definition, it's not equal to itself. That comes from the IEEE-754 standard that JavaScript numbers draw on. The "not a number" value is never equal to itself, even when the bits are an exact match. (Which they aren't necessarily in IEEE-754, it allows for multiple different "not a number" values.) Which is why this even comes up; all other values in JavaScript are equal to themselves, NaN is just special.

...am I missing some value in JavaScript that will return true for x !== x and false for x != x?

No, you're not. The only difference between !== and != is that the latter will do type coercion if necessary to get the types of the operands to be the same. In x != x , the types of the operands are the same, and so it's exactly the same as x !== x .

This is clear from the beginning of the definition of the Abstract Equality Operation:

ReturnIfAbrupt(x). ReturnIfAbrupt(y). If Type(x) is the same as Type(y), then Return the result of performing Strict Equality Comparison x === y. ...

The first two steps are basic plumbing. So in effect, the very first step of == is to see if the types are the same and, if so, to do === instead. != and !== are just negated versions of that.

So if Flanagan is correct that only NaN will give true for x !== x , we can be sure that it's also true that only NaN will give true for x != x .