Flattener

flattener.rb

class Flattener
  def initialize(nested_array)
    @nested_array = nested_array
  end

  def call
    @flattened_array = []
    flatten(nested_array)
    flattened_array
  end

  private

  attr_reader :flattened_array, :nested_array

  def flatten(array)
    array.each { |i| i.is_a?(Array) ? flatten(i) : flattened_array << i }
  end
end

flattener_spec.rb

require_relative 'flattener'

RSpec.describe Flattener do
  let(:flattener) { described_class.new nested_array }

  describe '#call' do
    context "with a small nested array" do
      let(:flattened_array) { [1,2,3,4] }
      let(:nested_array) { [[1,2,[3]],4] }

      it 'flattens nested arrays' do
        expect(flattener.call).to eq flattened_array
      end
    end

    context "with a large nested array" do
      let(:flattened_array) { %w(a b c d e f g) }
      let(:nested_array) { ["a", ["b", "c", ["d", [[["e"]]]]], "f", [["g"]]] }

      it 'flattens nested arrays' do
        expect(flattener.call).to eq flattened_array
      end
    end
  end
end

Check out the benchmarking for this as well.