Custom flatten method

flatten_array.rb

require 'minitest/autorun'

class FlattenArray
  def self.for(collection)
    new(collection).call
  end

  def initialize(collection)
    @collection = collection
  end

  def call
    flatten_array(@collection)
  end

  private
  def flatten_array(collection, cache = [])
    collection.each do |element|
      if element.is_a?(Array)
        flatten_array(element, cache)
      else
        cache << element
      end
    end
    cache
  end
end

class TestFlattenArray <  Minitest::Test
  def test_flatten_arry
    assert_equal(FlattenArray.for([[1,2,[3]],4]), [1,2,3,4])
  end

  def test_collection_is_not_mutated
    collection = [[1,2,[3]],4]
    FlattenArray.for(collection)
    assert_equal(collection, [[1,2,[3]],4])
  end
end