duplicated-code.md

Duplicated Code

Statements or expressions are copy-pasted, with minor variations, in several places.

What It Looks Like

def greet(message_type, name)
  case message_type
  when :email
    Email.mail "Greetings, #{name}"
  when :slack
    Slack.post "Greetings, #{name}"
  end
end

Why It Hurts

When the same concept is represented in multiple places, it's too easy for the duplicate representations to change out of sync. When that happens, it becomes unclear which variations are intentional and which are accidental. Chaos ensues.

How To Fix It

Follow the Flocking Rules.

Don't extract variables or methods for the identical portions of the duplicated code. While this may feel like you're making progress, it's a trap. You end up with code like this:

def greet(message_type, name)
  greeting = "Greetings, #{name}"
  case message_type
  when :email
    Email.mail greeting
  when :slack
    Slack.post greeting
  end
end

Here, we've replaced the message string with the variable greeting. This doesn't remove the duplication; it merely creates an alias for part of it. In the process, we introduced indirection, which makes the code harder to understand.

Caveats

Sandi Metz says, "Duplication is far cheaper than the wrong abstraction." A little bit of duplication is fine, especially if it's within one class or method. Writing more abstract code to remove the duplication can actually make code harder to understand when you haven't accurately identified the concept underlying the duplicated code.

When de-duplicating code, adhere to Preserve Whole Object. Keeping objects and values whole takes priority over removing all duplication.

Here's an example where whole values weren't preserved:

button_text = "Sign " +
  if current_user.nil?
    "In"
  else
    "Out"
  end

How much clearer is this?

button_text =
  if current_user.nil?
    "Sign In"
  else
    "Sign Out"
  end