Is it possible to detect NaN without using isNaN?

Question

Can I write a isNan() function with JavaScript? Can JavaScript check for sure that a variable is NaN without native isNaN function?

This is a interview question.

Answer 1

If it is a number, and is not equal to itself, it is NaN.

Note: As @pst has pointed out, the number constraint is unnecessary. NaN appears to be the only value in JS != itself.