U-X > undefined |
![]() ![]() ![]() |
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
.
When undefined
is converted to a string, it converts to the empty string.
The value undefined
is similar to the special value null
. In fact, 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 window. 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 window:
The value of x is x is undefined typeof (x) is undefined null and undefined are equal
Note: In the ECMA-262 specification, undefined
converts to the string "undefined"
, not the empty string. This is a difference between ActionScript and the ECMA-262 specification.
![]() ![]() ![]() |