Spooky variable declaration

Weird things can happen when you declare a variable within a conditional that you intend to use outside of it. Example:

if false
  x = 1
end
puts x # nil, even though the code declaring it was never executed
puts y # crash

See http://stackoverflow.com/questions/12928050/why-does-ruby-seem-to-hoist-variable-declarations-from-inside-a-case-statement-e .

I propose variables should never be declared for the first time within a conditional if you intend to use them outside.

Bad (works, but looks so scary):

if false
  x = 1
end
puts x # nil

Bad (works, but if you made a mistake in your logic you get the previous scenario, so it still looks alarming / requires too much inspection to figure out why it won't blow up):

if false
  x = 1
else
  x = nil
end
puts x # nil

Good (self-evidently safe in a way that doesn't rely on weird parser quirk):

x = nil # Declare the default value for the variable outside the conditional
if false
  x = 1
end
puts x # nil

Start by reading the linked Stack Overflow discussion and reviewing the style guide's existing guidance on variable declarations. Done means the guide has an agreed recommendation for variables first introduced inside conditionals, with the examples updated to make the intended behavior clear.