Local variables

Explanation

TL;DR

At first, Person#lastname is called.

Then a lastname local variable is created with a default value of nil as lastname = "Snow" is never executed due to the if false clause.

Finally, the final call to lastname ("#{firstname} #{lastname}") returns nil as it is the freshly created local variable that is called.

So Person#fullname returns "Jon " as Person#firstname returns Jon and the lastname local variable is equal to nil.

Disasm

Let's slightly modify our code example to see what happens behind the scenes

class Person
  def lastname = "Jones"

  def fullname
    lastname
    lastname = "Snow" if false
    lastname
  end
end

puts RubyVM::InstructionSequence.disasm(Person.new.method(:fullname))

OUTPUT

0000 putself                                                          (   5)[LiCa]
0001 opt_send_without_block                 <calldata!mid:lastname, argc:0, FCALL|VCALL|ARGS_SIMPLE>
0003 pop
0004 getlocal_WC_0                          lastname@0                (   7)[Li]
0006 leave                                                            (   8)[Re]

Firstly, we can see that a method with the mid (method identifier) lastname is called.

opt_send_without_block   <calldata!mid:lastname, argc:0, ...>

Then the lastname local variable is called

getlocal_WC_0   lastname@0

So once a local variable is referenced in the same scope, the local variable is created with the default value nil even if the referenced local variable is within a clause that is not executed like the if false one.

Voilà!