unless

An unless evaluates the then branch if its condition is falsey, and evaluates the else branch, if there’s any, otherwise. That is, it behaves in the opposite way of an if:

unless some_condition
  expression_when_falsey
else
  expression_when_truthy
end

# The above is the same as:
if some_condition
  expression_when_truthy
else
  expression_when_falsey
end

# Can also be written as a suffix
close_door unless door_closed?