This is a value representing invalid numeric values. It should be identical to the NaN value provided by the Global object in an ECMA-compliant implementation. Refer to the coverage of the NaN topic for full details.
However, it is generally considered unreliable to compare against NaN values with a simple equality test. To reliably test whether a numeric value is NaN or a good numeric value, use the isNaN() function and select an appropriate action according to its result.
ECMA 262 edition 2 - section - 188.8.131.52
ECMA 262 edition 3 - section - 184.108.40.206