Writing Scripts in Nashorn - Scripting in Java: Integrating with Groovy and JavaScript - page 162

The isNaN() Function
The isNaN() function has the following signature:
isNaN(value)
It returns true if value results in NaN when converted to a Number; that is, it returns
true if Number(value) return NaN . If the value represents a number, isNaN() returns
false . Notice that isNaN() does not only check if value is NaN ; it checks if value is a
number or not. NaN is not considered a number, so isNaN(NaN) returns true . If you are
interested in checking where value is NaN , use the expression value !== value . NaN is
the only value that is not equal to itself. So, value !== value will return true if and only if
value is NaN . The following code shows how to use the isNaN() function:
printf("isNaN(%s) = %b", NaN, isNaN(NaN));
printf("isNaN('%s') = %b", "123", isNaN('123'));
printf("isNaN('%s') = %b", "Hello", isNaN('Hello'));
printf("isNaN('%s') = %b", "97Hello", isNaN('97Hello'));
printf("isNaN('%s') = %b", "Infinity", isNaN('Infinity'));
printf("isNaN(%s) = %b", "1.89e23", isNaN(1.89e23));
var value = NaN;
if (value !== value) {
print("value is NaN")
}
else {
print("value is not NaN")
}
isNaN(NaN) = true
isNaN('123') = false
isNaN('Hello') = true
isNaN('97Hello') = true
isNaN('Infinity') = false
isNaN(1.89e23) = false
value is NaN