flatten_array_spec.rb

def flatten_array(input, base = [])
  if !input.is_a?(Array)
    raise 'When using `flatten_array`, an array must be supplied as the input.'
  end

  if !base.is_a?(Array)
    raise 'When using `flatten_array`, an array must be supplied as the base.'
  end

  input.each do |element|
    if element.is_a?(Array)
      flatten_array(element, base)
    else
      base << element
    end
  end

  base
end

describe 'flatten_array' do
  context 'when input is an array' do
    context 'and it contains nested arrays' do
      it 'returns a flattened array' do
        array = [[1, 2, [3]], 4]
        new_array = flatten_array(array)
        expect(new_array).to eq([1, 2, 3, 4])
      end
    end

    context 'and it does not contain nested arrays' do
      it 'returns a flattened array' do
        array = [1, 2, 3, 4]
        new_array = flatten_array(array)
        expect(new_array).to eq([1, 2, 3, 4])
      end
    end
  end

  context 'when input is not an array' do
    it 'raises an exception' do
      expect {
        flatten_array('BOOM!')
      }.to raise_error('When using `flatten_array`, an array must be supplied as the input.')
    end
  end

  context 'when base is not an array' do
    it 'raises an exception' do
      expect {
        flatten_array([1], 'BOOM!')
      }.to raise_error('When using `flatten_array`, an array must be supplied as the base.')
    end
  end
end