MACROMEDIA FLASH MX 2004 - ACTIONSCRIPT - Page 740

740 Chapter 12: ActionScript Dictionary
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.
Example
In this 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");
}
The following result is displayed in the Output panel.
The value of x is undefined
x is undefined
typeof (x) is undefined
null and undefined are equal