Javascript [Electronic resources] : The Definitive Guide (4th Edition) نسخه متنی

3.8 undefined

Another
special value used occasionally by JavaScript is the
undefined value returned when you use either a
variable that has been declared but
never had a value assigned to it, or an object
property
that does not exist. Note that this special
undefined value is not the same as
null.

Although null and the undefined
value are distinct, the == equality operator considers them to
be equal to one another. Consider the following:

my.prop == null

This comparison is true either if the
my.prop property does not exist or if it does
exist but contains the value null. Since both
null and the undefined value
indicate an absence of value, this equality is often what we want.
However, if you truly must distinguish between a
null value and an undefined
value, use the === identity operator or the
typeof operator (see Chapter 5
for details).

Unlike null, undefined is not a
reserved word in JavaScript. The ECMAScript v3 standard specifies
that there is always a global
variable named undefined whose initial value is
the undefined value. Thus, in a conforming
implementation, you can treat
undefined
as a keyword, as long as you don't assign a value to the
variable.

If you are not sure that your implementation has the
undefined variable, you can simply declare your
own:

var undefined;

By declaring but not initializing the variable, you assure that it
has the undefined value. The
voidChapter 5) provides another way to obtain the
undefined value.