Simply returns true if the argument coerces to NaN and false otherwise.
var n = ToNumber(number);
return n !== n; // see below
The specification notes that a reliable way to for Ecmascript to test whether a variable is NaN is to check x !== x. It says this can only be false if x is a NaN. Remember that NaN is the only value that is not equal to itself. The identity check ensures coercion can't screw this assumption up.