Refactoring Ruby: Extract Variable

Let’s take a look at a simple way to add clarity to your Ruby methods: Extract Variable or Introduce Explaining Variable. It's probably one of the least invasive refactoring methods. However, don’t let the simplicity fool you; any clarity or explicitness gained is well worth the change.

When you have complex expressions grouped together in a method, it's best to describe them as clearly and verbosely as necessary using the Extract Variable technique. Let's apply this approach to the Movie#total_price method in the code example below:

class Movie
  TAX_PERCENTAGE = 0.5

  def initialize(price:, discount_percentage: 0)
    @price = price
    @discount_percentage = discount_percentage
  end

  def total_price
    # BAD
    price + (price * TAX_PERCENTAGE) - (price * (discount_percentage / 100.0))
  end

  private

  attr_reader :price, :discount_percentage
end

class Movie
  TAX_PERCENTAGE = 0.5

  def initialize(price:, discount_percentage: 0)
    @price = price
    @discount_percentage = discount_percentage
  end

  def total_price
    # GOOD
    tax = price * TAX_PERCENTAGE
    discount = price * (discount_percentage / 100.0)

    price + tax - discount
  end

  private

  attr_reader :price, :discount_percentage
end

The bad version looks like:

def total_price
  # BAD
  price + (price * TAX_PERCENTAGE) - (price * (discount_percentage / 100.0))
end

While you can deduce that this method returns the total price of a movie, it takes too much effort to understand how it does so.

To improve this method, we'll apply the Extract Variable pattern:

def total_price
  # GOOD
  tax = price * TAX_PERCENTAGE
  discount = price * (discount_percentage / 100.0)

  price + tax - discount
end

In the improved "good" version, we introduce two explaining variables: tax and discount.

Now, at a glance we know the total price of a movie consists of price + tax — discount, and we now have clear, concise descriptions of those complex expressions.

By applying the Extract Variable refactoring pattern, we've made the code more readable and understandable, enhancing its overall clarity.