MACROMEDIA FLASH 8-FLASH LITE 2.X ACTIONSCRIPT LANGUAGE - Page 31

Constants 31
In files published for Flash Player 6 or earlier, the value of Number(undefined) is 0. In files
published for Flash Player 7 or later, the value of
Number(undefined) is NaN.
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.
Availability: ActionScript 1.0; Flash Lite 1.1
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 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 displayed in the Output panel.
The value of x is undefined
x is undefined
typeof (x) is undefined
null and undefined are equal