Mixin for extending ruby array with :recursive_flatten method (as supposedly it should be used often). Used default tests (instead of rspec) for simplicity

flatten_array.rb

# frozen_string_literal: true

class Array
  module Mixins
    # Mixing for providing flatten method into Array
    module Flatten
      # Returns a flatten copy of array
      # Example: [[1,2,[3]],4].recursive_flatten => [1, 2, 3, 4]
      def recursive_flatten(result: [])
        each do |element|
          if element.is_a?(Array)
            element.recursive_flatten(result: result)
          else
            result << element
          end
        end

        result
      end
    end
  end
end
Array.include Array::Mixins::Flatten

test_flatten_array.rb

require 'test/unit'
require_relative './array_flatten'

class TestArrayFlattenMixin < Test::Unit::TestCase
  def test_empty_array
    subject = [].recursive_flatten
    assert_equal(subject, [])
  end

  def test_simple_array
    subject = [2, 3, 4, 5].recursive_flatten
    assert_equal(subject, [2, 3, 4, 5])
  end

  def test_array
    subject = [[1, 2, [3]], 4].recursive_flatten
    assert_equal(subject, [1, 2, 3, 4])
  end
end