This is a built-in function to check for the Not-a-Number value. Because it is a member of the Global object, and the Global object is permanently in the prototype inheritance chain, you don't need to identify which object the function belongs to.
Applies the internal ToNumber operator to its argument and returns true or false depending on whether the value is a number or not.
The values may not always yield the result you expect.
These all yield a false value:
These all yield a true value:
The result is true for valid numeric values and false for invalid numerics.
This is not very useful on MSIE version 3.02 since it cannot understand what a NaN is in the first place.
Comparing the NaN value with anything using the == operator will always yield the Boolean false value.
ECMA 262 edition 2 - section - 220.127.116.11
ECMA 262 edition 3 - section - 18.104.22.168