Applescript - return value from 'do Javascript'

I'm running a javascript via Applescript, and I want to return a value back to the calling applescript. When I try the following test:

tell application "Adobe Illustrator"

set theScript to "return 'Test';"

set theResult to do Javascript theScript

end tell

I get a message "'Illegal 'return' outside of a function body".

Is there a way to return a value back to the applescript?