flattenizer.rb

# Method which flattens the array:[[1,2,[3]],4] -> [1,2,3,4]
module Flattenizer
  def flattenize array
    return [array] unless array.kind_of?(Array) # result of method is always array
    result = []
    array.each do |item|
      flattenize(item).each do |r| # ensure that result will not be nested
        result << r
      end
    end
    result
  end
end

flattenizer_test.rb

require "minitest/autorun"
require './flattenizer'
include Flattenizer

class TestFlattenize < Minitest::Test
  def test_not_nested_array
    assert_equal [1, 2, 3], flattenize([1, 2, 3])
  end

  def test_nested_array
    assert_equal [1, 2, 3], flattenize([1, [2, [3]]])
  end

  def test_if_integer_is_converted_into_array
    assert_equal [1], flattenize(1)
  end

  def test_if_method_works_with_empty_array
    assert_equal [], flattenize([])
  end
end

ruby -Ilib:test flattenizer_test.rb

To test it in console ;)