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.

Undefined Attribute Values

Another application of NULL values is when class attributes and associations are undefined. If an object is created of a class that contains attributes that have no initial values, these attributes are set to NULL. You can use the boolean operation exists() to test whether attributes 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;