MACROMEDIA FLASH MX, Reference

Page: 989 / 1104

undefined 989
undefined
Availability
Flash Player 5.
Usage
undefined
Parameters
None.
Returns
Nothing.
Description
A special value, usually used to indicate that a variable has not yet been assigned a value.
A reference to an undefined value returns the special value
undefined
. The ActionScript code
typeof(undefined)
returns the string
"undefined"
. The only value of type
undefined
is
undefined
.
In files published for Flash Player 6 or earlier, the value of
undefined.toString()
is
"" (an empty string). In files published for Flash Player 7 or later, the value of
undefined.toString()
is
undefined
.
The value
undefined
is similar to the special value
null
. When
null
and
undefined
are
compared with the equality (
==
) operator, they compare as equal. However, when
null
and
undefined
are compared with the strict equality (
===
) operator, they compare as not equal.
Example
In the following example, the variable
x
has not been declared and therefore has the value
undefined
. In the first section of code, the equality operator (
==
) compares the value of
x
to the
value
undefined
, and the appropriate result is sent to the Output panel.In the second section of
code, the equality (
==
) operator compares the values
null
and
undefined
.
// x has not been declared
trace("The value of x is "+x);
if (x == undefined) {
trace("x is undefined");
} else {
trace("x is not undefined");
}
trace("typeof (x) is "+typeof (x));
if (null == undefined) {
trace("null and undefined are equal");
} else {
trace("null and undefined are not equal");
}
CHAPTER 2
ActionScript Language Reference