Problems Addressed

Don't Repeat Yourself
Copy/Paste Programming
No single source of truth

Related Code Smells

Code Smell 46 - Repeated Code

Maxi Contieri ・ Dec 8 '20

#oop #codenewbie #tutorial #webdev

Code Smell 11 - Subclassification for Code Reuse

Maxi Contieri ・ Oct 30 '20

#oop #codenewbie #tutorial

Code Smell 232 - Reusable Code

Maxi Contieri ・ Nov 25 '23

#javascript #programming #webdev #tutorial

Context

Duplicated code is a severe code smell, it leads to maintainability problems and ripple effects.

Start by identifying behavior duplication.

Once you find it, you will extract it into reusable functions or classes, reducing redundancy, creating a single source of truth, and simplifying future updates.

Behavior duplication is a sign of a missing abstraction you need to create.

As always, you should search for it in the real world.

Refactoring isn't a one-time event; it's an ongoing process that should be integrated into your development workflow.

Steps

Make a contextual copy of the repeated code
Parametrize what is different
Invoke the abstraction
Find a real-world metaphor for the abstraction

(This is the harder and not mechanical step)

Sample Code

*(This is actual code generated by Google Gemini)

See a complete explanation in this talk*

Before

<?php

class AccessControlPanel {

  private $users = [];

  public function createRegularUser($username, $password, $email) {
    $user = [
      "username" => $username,
      "email" => $email,
      "type" => $this->regularUserRole(),
      "creationDate" => $this->timeSource->currentTimestamp(),
      "needsToChangePassword" = $this->needsToChangePassword(),
      "loginPolicy" => $this->userLoginPolicy()
    ]
    $this->users[] = $user;
    $this->addCreationToJournal($user);
  }

   public function createAdminUser($username, $password, $email) {
    $user = [
      "username" => $username,
      "email" => $email,
      "type" => $this->regularUserRole(),
      "creationDate" => $this->timeSource->currentTimestamp(),
      "needsToChangePassword" = $this->needsToChangePassword(),
      "loginPolicy" => $this->adminUserLoginPolicy()
    ]
    $this->users[] = $user;
    $this->addCreationToJournal($user);
    return $user;
  }
} 

?>

After

<?php

class AccessControlPanel {

  private $users = [];

  // 1.  Make a contextual copy of the repeated code

  private function createUser(
     $username, 
     $password,
     $email, 
     $role, 
     $loginPolicy) {
    $user = [
      "username" => $username,
      "email" => $email,
      "type" => $role,
      "creationDate" => $this->timeSource->currentTimestamp(),
      "needsToChangePassword" => $this->needsToChangePassword(),
      "loginPolicy" => $loginPolicy
    ];
    $this->users[] = $user;
    $this->addCreationToJournal($user);
    return $user;
  }

  // 2. Parametrize what is different (in this case $role and $loginPolicy)

  public function createRegularUser($username, $password, $email) {
    // 3. Invoke the abstraction
    return $this->createUser(
      $username,
      $password,
      $email, 
      $this->regularUserRole(),
      $this->userLoginPolicy());
  }

  public function createAdminUser($username, $password, $email) {
    return $this->createUser(
      $username,
      $password,
      $email,
      $this->adminUserRole(), 
      $this->adminUserLoginPolicy());
  }

  // 4. Find a real-world metaphor for the abstraction
  // private function createUser(
  // $username, 
  // $password,
  // $email, 
  // $role, 
  // $loginPolicy)
}

?>