JavaScript 2.0-The Complete Reference, Second ... - freecodingtutorial

The other important special value is NaN, which means ―not a number.‖ Numeric data takes on
this value when it is the result of an undefined operation. Common examples of operations that
result in NaN are dividing zero by zero, taking the sine of Infinity, and attempting to add
Infinity to –Infinity. The NaN value is also sticky, but unlike the infinite values it never
compares equal to anything. Because of this, you must use the isNaN() method or compare the
value to itself to determine if a value is NaN. The isNaN() method returns a Boolean indicating
whether the value is NaN. This method is so important that it is a property of the Global object,
so it can be called directly in your scripts. Comparing the value to itself will indicate whether the
value is NaN because it is the only value that does not compare equal to itself!
The following example illustrates the use of both techniques:
var x = 0 / 0; // assign NaN to x
if (x != x) // check via self-equality
{
// do something
}
if (isNaN(x)) // check via explicit call
{
// do something
}
Table 3-1 summarizes these special types.
Table 3-1: Summary of Special Numeric Data Values
Special Value Result of Comparisons Sticky?
Infinity, –Infinity Number too large or
small to be represented
All Infinity values
compare equal to
each other
NaN Undefined operation NaN never
compares equal to
anything, even itself
Yes
Yes