Flattens an Array without using the built-in Array#flatten method (For testing it requires spec)

array_flattener.rb

module ArrayUtils
  extend self

  # Converts a nested array to a flatten array without using the built-in method Array#flatten
  # It doesn't remove nil
  #
  # If the passed parameter is not an array, the same object is returned
  #   to_flat_array(nil) #=> nil
  #   to_flat_array({ a: 'something }) #=> {:a=>"something"}
  # If the passed parameter is a nested array, a flatten array is returned
  #   to_flat_array([[1, 2, [3]], 4]) #=> [1, 2, 3, 4]
  # If the passed parameter is an array, and contains a non array element inside, that element is not changed
  #   to_flat_array([1, 2, 3, 4, { a: 'a' }]) #=> [1, 2, 3, 4, {:a=>"a"}]
  def to_flat_array(array = [])
    # If the passed object is not an array, the same object must be returned
    return array unless array.is_a?(Array)
    array.inject([]) do |result, element|
      result.push *(element.is_a?(Array) ? to_flat_array(element) : [element])
    end
  end
end

array_flattener_spec.rb

require_relative 'array_flattener.rb'
require 'rspec'

RSpec.describe ArrayUtils do
  context '::to_flat_array' do
    context 'when an array is passed' do
      context 'and it has nested arrays inside' do
        it 'must return a unique flatten array' do
          expect(ArrayUtils::to_flat_array([[1, 2, [3]], 4])).to eq([1, 2, 3, 4])
        end
      end

      context 'and it has no nested arrays inside' do
        it 'must return the same array' do
          expect(ArrayUtils::to_flat_array([1, 2, 3, 4])).to eq([1, 2, 3, 4])
        end
      end
    end

    context 'when a non-array is passed' do
      it 'must return the same object' do
        expect(ArrayUtils::to_flat_array({ a: 'something' })).to eq({ a: 'something' })
      end
    end

    context 'when nil is passed' do
      it 'must return nil' do
        expect(ArrayUtils::to_flat_array(nil)).to be_nil
      end
    end
  end
end