Assignment still proceeds even with undefined local variable or method

This week I had a really horrible bug. One spec in our suite in our CI knocked out every subsequent spec after skipping. The spec works in isolation. just not on CI and not in a full suite run.

RSpec.shared_contexts 'with some feature' do
  skip('This is not available on CI') if ENV['CI']
  original_driver = Capybara.javascript_driver

  Capybara.javascript_driver = :something_else
ensure
  Capybara.javascript_driver  = original_driver
end

RSpec.describe 'something' do
  include_context 'with some feature'
  it do
    visit '/'
  end
end

Turns out our Capybara.javascript_driver ended up being nil, and our test suite fell over because CI doesn’t support non-headless Chrome. And the reason…

I would appear that variable assignment in Ruby still assigns nil if we attempt to assign a variable from another variable or method that is undefined.

irb(main):001> a = some_method_or_var_that_is_not_defined
(irb):1:in `<main>': undefined local variable or method `some_method_or_var_that_is_not_defined' for main (NameError)

a = some_method_or_var_that_is_not_defined
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
	from <internal:kernel>:187:in `loop'
	from /Users/rl/.asdf/installs/ruby/3.3.0/lib/ruby/gems/3.3.0/gems/irb-1.14.0/exe/irb:9:in `<top (required)>'
	from /Users/rl/.asdf/installs/ruby/3.3.0/bin/irb:25:in `load'
	from /Users/rl/.asdf/installs/ruby/3.3.0/bin/irb:25:in `<main>'
irb(main):002> a
=> nil

Who knew? I didn’t.