Custom flatten implementation in ruby

ruby_custom_flatten.rb

class FlattenTest
  def self.custom_flatten(array, result = [])
    array.each do |item|
      if item.is_a? Array
        custom_flatten(item, result)
      else
        result << item
      end
    end
    result
  end
end

RSpec.describe FlattenTest do

  context '.custom_flatten' do
    it 'should flatten array of arrays' do
      expected = [1, 2, 3, 4, 5, 6]
      expect(FlattenTest.custom_flatten([[1, 2], 3, [4, 5, 6]])).to eq expected
    end

    it 'should flatten nested array of arrays' do
      expected = [1, 2, 3, 4, 5, 6, 7, 8, 9]
      expect(FlattenTest.custom_flatten([[1, 2], 3, [4, 5, [6, [7, 8], 9]]])).to eq expected
    end

    it 'should return already flatten array' do
      expected = [1, 2, 3, 4]
      expect(FlattenTest.custom_flatten(expected)).to eq expected
    end

  end
end