As an expression

The value of an if is the value of the last expression found in each of its branches:

a = if 2 > 1
      3
    else
      4
    end
a # => 3

If an if branch is empty, or it’s missing, it’s considered as if it had nil in it:

if 1 > 2
  3
end
# The above is the same as:
if 1 > 2
  3
else
  nil
end
# Another example:
if 1 > 2
else
  3
end
# The above is the same as:
if 1 > 2
  nil
else
  3
end