NULL Values
Variable values can evaluate to NULL. This value symbolizes a non-valid resp. not-existing value of an object. For example, if a boolean value could not be calculated since an error occurred during its calculation, it might be set to NULL indicating that it is not known whether the correct value is true or false.
You can also set any variable or property to NULL using set aVariable = NULL;.
Undefined Object Property Values
Another application of NULL values is when object properties are undefined. If an object is created of a class that contains properties that have no initial values, these properties are set to NULL. You can use the boolean operation exists() to test whether properties or associations exist or not.
aVariable.exists()
Undefined Return Values
Action script expressions may also return NULL. Use logical operators to determine whether an operation returns a NULL value, e.g. for a decision, you could add the following conditional statement as a guard expression:
aParameterString.findString("LanguageFlag") != NULL;