Refactoring: Refactorings/Introduce Explaining Variable

Use a variable name to explain a complicated expression

Mechanics

1. Assign a temporary variable to the result of part of the complex expression
2. Replace the result part of the expression with the value of the temp

   Note: If the result part of the expression is repeated, you can replace the repeats one at a time

3. Test
4. Repeat for other parts of the expression

Example

Before

def price
  # price is base price - quantity discount + shipping
  return @quantity * @item_price - [0, @quantity - 500].max * @item_price * 0.05 + [@quantity * @item_price * 0.1, 100.0].min
end

After

Note: Could also use Extract Method to turn base_price, quantity_discount, and shipping into function calls

def price
  base_price = @quantity * @item_price
  quantity_discount = [0, @quantity - 500].max * @item_price * 0.05
  shipping = [base_price * 0.1, 100.0].min
  return base_price - quantity_discount + shipping
end

Using Move Method Instead

(This is Fowler's preferred solution)

def price
  base_price - quantity_discount + shipping
end

def base_price
  @quantity * @item_price
end

def quantity_discount
  [0, @quantity - 500].max * @item_price * 0.05
end

def shipping
  [base_price * 0.1, 100.0].min
end