User Guide

224 Chapter 5: ActionScript Core Language Elements
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.
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 log file. 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 written to the log file.
The value of x is undefined
x is undefined
typeof (x) is undefined
null and undefined are equal
CHAPTER 5
ActionScript Core Language Elements