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

Note Some Netscape browsers do not support this property so they will always display the
second message (―Persistent cookies are not enabled‖). This is because Boolean
variables are assigned a default value of false if none is assigned.
Undefined and Null
The undefined type is used for variables or object properties that either do not exist or have not
been assigned a value. The only value an undefined type can have is undefined. For example,
declaring a variable without assigning it a value,
var x;
gives x the undefined type and value. Accessing a nonexistent object property,
var x = String.noSuchProperty;
also results in the assignment of undefined to x.
The null value indicates an empty value; it is essentially a placeholder that represents
―nothing.‖ The distinction between undefined and null values is tricky. In short, undefined
means the value hasn‘t been set, whereas null means the value has been set to be empty.
Why on earth would the designers of JavaScript permit such a confusing distinction? Without
getting into the arcane details, there are actually certain cases where it can be useful,
particularly if you‘re using the object-oriented features of the language. For example, suppose
you‘re writing an object whose functionality for a method called doFancyStuff() depends upon
a feature offered only by some browsers. You might do some browser detection and define the
function doFancyStuff() appropriately if the browser is one that supports the required feature.
But if the user has an unrecognized browser, you might set doFancyStuff to null to indicate
the method is unavailable. In this way, you can distinguish between the case that the feature is
supported (doFancyStuff is a function), the case that the feature isn‘t supported
(doFancyStuff is null), and the case that the browser detection code hasn‘t been run
(doFancyStuff is undefined).
There is one further wrinkle to be aware of: the null value is defined as an empty object.
Because of this, using the typeof operator on a variable holding null shows its type to be
object. In comparison, the type of undefined data is undefined.
Distinguishing Between Null and Undefined Data
JavaScript provides the null keyword to enable comparison and assignment of null values.
Unfortunately, the undefined keyword exists only in modern browsers (Netscape 6+ and
Internet Explorer 5.5+). The resolution lies in the fact that null and undefined values compare
equal. So you can check for invalid values by comparing to null. For example, given the
declarations,
var x;
var y = null;
the following comparisons are true:
if (x == null)