In the standalone mongosh and mongo shell, an undefined result is ignored:

> var foo = '123'

Current behaviour in Compass 1.22.1 is to display "undefined":

> var foo = '123'
< undefined

This outcome is confusing for operations that evaluate to undefined rather than providing a meaningful result (for example, variable declaration). It appears as though the operation may have failed (or be unsupported) when the result code is actually irrelevant.