array_tools.rb

class ArrayTools
  def self.flat_array(array = [])
    array.each_with_object([]) do |item, flat_array|
      next if item.kind_of? String
      flat_array.push *(item.kind_of?(Array) ? flat_array(item) : item)
    end
  end
end

array_tools_spec.rb

RSpec.describe ArrayTools do

  describe '.flat_array' do
    it 'flattens 1 level nested array' do
      expect(Group.flat_array([1, 2, 3, [9, 8], 7])).to eq [1, 2, 3, 9, 8, 7]
    end
    it 'flattens an empty array' do
      expect(Group.flat_array([])).to eq []
    end
    it 'flattens a nested empty array' do
      expect(Group.flat_array([1, []])).to eq [1]
    end
    it 'flattens a 2 leves nested array' do
      expect(Group.flat_array([1, [[3, 4]]])).to eq [1, 3, 4]
    end
    it 'flattens arrays with string' do
      expect(Group.flat_array(['sbrubles', 2, 3, [9, 8], 7])).to eq [2, 3, 9, 8, 7]
      expect(Group.flat_array([1, 2, 3, ['bla', 9, 8], 7])).to eq [1, 2, 3, 9, 8, 7]
    end
  end
end