HTML and CSS Reference
In-Depth Information
Figure 3.2
Output from Example 3.3.
Null
and
Undefined
.
The difference between null and undefined is a little subtle.
The
null
keyword represents “no value,” meaning “nothing,” not even an empty string
or zero. It is a type of JavaScript object (see Chapter 8, “Objects”). It can be used to ini-
tialize a variable so that it does not produce errors or to clear the value of a variable, so
that there is no longer any data associated with that variable, and the memory used by
it is freed. When a variable is assigned
null
, it does not contain any valid data type.
A variable that has been declared, but given no initial value, contains the value
unde-
fined
and will produce a runtime error if you try to use it. (If you declare the variable
and assign
null
to it,
null
will act as a placeholder and you will not get an error.) The
word
undefined
is not a keyword in JavaScript. If compared with the == equality opera-
tors,
null
and
undefined
are equal, but if compared with the identity operator, they are
not identical (see Chapter 5, “Operators”).
EXAMPLE
3.4
<html>
<head>
<title>The typeof Operator with Null and Undefined</title>
</head>
<body bgColor="gold">
<big>
<script type="text/javascript">
document.write("<em>null</em> is type "+
1
typeof(null)
,"<br />");
document.write("<em>undefined</em> is type "+
2
typeof(undefined)
,"<br />");
</script>
</big>
</body>
</html>
EXPLANATION
1
The
null
keyword is a type of object. It is a built-in JavaScript object that contains
no value.
2
Undefined is returned when a variable has been given no initial value or when the
void
operator is used (see Table 5.19 on page 120). See output in Figure 3.3.
