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Guard Clauses vs. Nested Conditionals

anthonyharvey profile image Anthony Harvey Originally published at anthonygharvey.com ・2 min read

TLDR; a guard clause is a premature return (early exit) that "guards" against the rest of your code from executing if it's not necessary (based on criteria you specify).


Soon after I started my career as a Ruby on Rails developer I learned about guard clauses and how they can improve code readability.

Let's take a look at an example method with a standard if/else statement.

def example_method(parameter)
  if parameter == true
    # awesome code here
  else
    return nil
  end
end
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In this method we're doing something that's common: evaluating a parameter and:

  • executing some code if our expectations are met OR
  • executing some other code if our expectations are not met.

In this case:

  • if the parameter is true we execute some "awesome code"
  • if not, we just return nil

This method can be shortened to only two lines of code by using a guard clause.

def example_method(parameter)
  return nil unless parameter == true #<-- guard clause
  # awesome code here
end
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unless method

This was as short example to illustrate the concept of a guard clause. Now let's look at an example where a guard clause can help eliminate a code smell: nested conditionals.


def print_shipping_label(order)
  if order.outstanding_payments?
    nil
  else
    if order.address_incomplete?
      order.send_address_reminder
    else
      if order.standard_shipping?
        order.print_standard_shipping_label
      else
        order.print_priority_shipping_lable
      end
    end
  end
end
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This code is hard to read and follow what's happening in my opinion (just look at how many levels of indention are in the nested if/else statements!)

Refactoring Nested Conditionals Using Guard Clauses

We can use guard clauses to refactor the nested if/else statements and make the method more readable and easier to follow.

def print_shipping_label2(order)
  return nil if order.outstanding_payments?
  return order.send_address_reminder if order.address_incomplete?
  return order.print_standard_shipping_label if order.standard_shipping?
  order.print_priority_shipping_label
end
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I like guard clauses for these reasons, but you should use them where it makes sense for you and your team.

For example, some languages require explicit resource management and having a single exit point from a method would make sense to prevent memory leaks. However Ruby has garbge collection to automatically collect unused objects.


Takeaway:

Guard clauses guard (protect) the rest of your code from executing if not necessary (based on certain criteria) and are placed at the top of a method.

Resources:

  1. Refactoring.com - Replace Nested Conditional with Guard Clauses
  2. Refactoring.guru - Replace Nested Conditional with Guard Clauses

This article was originally published on anthonygharvey.com on July 28th, 2019.

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