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VBScript Tutorial

Keyword	Description
Empty	Used to indicate an uninitialized variable value. A variable value is uninitialized when it is first created and no value is assigned to it, or when a variable value is explicitly set to empty. Example: Dim x 'the variable x is uninitialized! x="ff" 'the variable x is NOT uninitialized anymore x=Empty 'the variable x is uninitialized! Note: This is not the same as Null!!
IsEmpty	Used to test if a variable is uninitialized. Example: If (IsEmpty(x)) 'is x uninitialized?
Nothing	Used to indicate an uninitialized object value, or to disassociate an object variable from an object to release system resources. Example: Set myObject=Nothing
Is Nothing	Used to test if a value is an initialized object. Example: If (myObject Is Nothing) 'is it unset? Note: If you compare a value to Nothing, you will not get the right result! Example: If (myObject = Nothing) 'always false!
Null	Used to indicate that a variable contains no valid data. One way to think of Null is that someone has explicitly set the value to "invalid", unlike Empty where the value is "not set". Note: This is not the same as Empty or Nothing!! Example: x=Null 'x contains no valid data
IsNull	Used to test if a value contains invalid data. Example: if (IsNull(x)) 'is x invalid?
True	Used to indicate a Boolean condition that is correct (True has a value of -1)
False	Used to indicate a Boolean condition that is not correct (False has a value of 0)