set variable to null

It depends on what you're trying to do.

If you're trying to erase a variable's value but leave it defined, just set it to any random value.

If you're actually trying to make a variable undefined, use:

set A to (beep 0)

If you want to keep the variable defined but give it the null value, use:

set A to null

(49395)

set X to ""

This will set X to an empty string - not quite the same thing as 'null' since it still has a class ('text').

You can:

set X to null

which sets it to the 'null' class object - it still has a 'class' but no value.

To completely void a variable, use:

set X to missing value

now it doesn't have any content, not even a class. Whether or not you need this level of emptiness depends on your application.

now it doesn't have any content, not even a class

That's not true; both null and missing value have the 'class' class. In order to completely void it, it needs to be set to the output of a command or handler which doesn't return a result, not even 'null' or 'missing value'. The 'beep 0' I used above is an example of one.

(49396)

Then you better use true and false boolean values.

Hello

When you're managing two states, a straightforward way is to use boolean true and false as the codes below. Of course you can use 'yes' and 'no', 1 and 0, etc. as you like.

set test to some item of {0, 1}
if test = 1 then
 set x to true
else
 set x to false
end if
return {test, x}

set x to false
set test to some item of {0, 1}
if test = 1 then set x to true
return {test, x}

set test to some item of {0, 1}
set x to test = 1
return {test, x}

As for documentation, you may read The AppleScript Language Guide.
http://developer.apple.com/documentation/AppleScript/Conceptual/AppleScriptLangG uide/

Cheers,
H