Only set var if expression returns a defined result?

[dvlsg]

Is there a way to only set a var if an expression returns something other than undefined?

For example, I have a collection-level post-request var expression to pull a value off of the body, say res.body.token -> varToken. This res.body.token value exists for only some of the responses in the collection. Is there a way to keep varToken from being overwritten when res.body.token doesn't exist, using an expression in the Vars tab?

Not a huge deal, I know I can just use a collecton level script instead. But curious if there's a shorthand expression I could have used with the Vars tab instead.