Flash CS4 Professional ActionScript 2.0

undefined constant

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 String(undefined) is "" (an empty string). In files published for Flash Player 7 or later, the value of String(undefined) is "undefined" (undefined is converted to a string).

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 Player 5

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");
}

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