2.2.2 [ECMA-262/5] Section 8.5, The Number Type
The specification states:
In some implementations, external code might be able to detect a difference between various Not-a-Number values, but such behaviour is implementation-dependent; to ECMAScript code, all NaN values are indistinguishable from each other.
IE9 Mode, IE10 Mode, and IE11 Mode (All Versions)
NaN values are not normalized to the same value.