void data type

The void data type contains only one value, undefined. In previous versions of ActionScript, undefined was the default value for instances of the Object class. In ActionScript 3.0, the default value for Object instances is null. If you attempt to assign the value undefined to an instance of the Object class, Flash Player will convert the value to null. You can only assign a value of undefined to variables that are untyped. Untyped variables are variables that either lack any type annotation, or use the asterisk (*) symbol for its type annotation.