flatten.rb

def flatten(arr, into: [])
  arr.each do |el|
    case el
    when Array then flatten(el, into: into)
    else into.push(el)
    end
  end

  into
end

flatten_spec.rb

require 'rspec'
require_relative 'flatten'

describe 'flatten' do
  it 'flattens arrays' do
    expect(flatten([[1, 2, [3]], 4])).to eq([1, 2, 3, 4])
    expect(flatten([[1], [[2]]])).to eq([1, 2])
  end
end

Gemfile

source 'https://rubygems.org'

gem 'rspec'