Many of our daily lives are tied to websites, from shopping websites to educational and banking websites, etc. All these websites are created with the aim of satisfying user needs and creating a positive user experience. And a positive user experience ultimately benefits the company, organization, or business.

But what happens behind the scenes of these websites? What happens in the JavaScript code of the website that makes the website more efficient? What principles should web developers and programmers follow so that the final website works smoothly both technically and user-wise?

In this article, we introduce the principles of SOLID. The principles programmer and developer of the website and software can design a developable, maintainable, and generally understandable product by following them.

What is SOLID?

In programming, the SOLID concept refers to a set of software design principles and rules. These principles are defined to build maintainable, expandable, and understandable software.

The word SOLID stands for the following five concepts:

  • S: Single Responsibility Principle
  • O: Open-Closed Principle
  • L: Liskov Substitution Principle
  • I: Interface Segregation Principle
  • D: Dependency Inversion Principle

SOLID principles help programmers to make the final product (website, application, or software) maintainable, extensible and testable, and easier to read. In other words, programmers and developers write programs that are better managed by following these principles.

Also, new requirements can be easily added in the future. Therefore, SOLID principles are independent of programming languages and can be used in all object-oriented programming languages such as C, C#, Java, Java Script, Python, etc.

How to apply and implement these principles may be slightly different depending on the characteristics and features of each programming language. But the goals and concept of SOLID principles are the same for all programming languages.

In the following, we will introduce the five principles of SOLID.

1. Single Responsibility Principle

The first principle of SOLID is known as the single responsibility principle. This principle states that each piece of code should be responsible for only one task, and no more. A code consists of different parts. Each department has one and only one specific responsibility. And by putting these parts together, the final code will be simpler and more organized, and it will be easier to change it in the future.

Suppose you want to write a simple class to manage the sales of an online store. You have a class called “OrderManager” whose task is to manage orders. Using the single-responsibility principle, you can split the “OrderManager” class into several separate classes, such as “OrderValidator” to validate orders, “OrderProcessor” to process orders, and “OrderNotifier” to notify about orders.

Using the Single Responsibility Principle, each class has its responsibility. And the change in one class has a small and limited effect on other classes.

If the input of one function changes, the rest of the classes are independently unaffected. Unless there is a change in the overall structure or functionality of the program. For example, if the order data structure changes, it is likely that the OrderValidator class needs to be modified to validate the new data. But this change will not affect other classes like `OrderProcessor` or `OrderNotifier`.

With this division, each class has its responsibility. This makes the code more extensible and modifiable, as well as easier to test and maintain.

2. Open-Closed Principle

The second principle of SOLID states that the program should be open to development. It means that its function can be changed and new features can be added to it. But it must remain closed for external changes. External changes are changes that are applied to the program and lead to changes in the behavior of the program on the user side or other systems.

For example, let’s say you have a sales management program that can add new products to the system. If you follow the open/closed principle, the app should be able to easily add new products. In other words, the application should be open so that we can easily make changes to it and add new features to it.

Also, the program must remain closed to external changes. This means that the changes we make in the program should not have negative and unexpected effects on other parts of the program.

Following the principle of openness/closedness, the program will be expandable and flexible. And at the same time, it will maintain its stability and correct performance.

3. Liskov Substitution Principle

Liskov’s substitution principle states that the objects of any child class that inherits from a parent class must be replaceable with objects of the parent class. That is, it can be used instead of the parent class without causing errors or changes in the program’s performance.

Let’s say you have a pet management app. In this program, you have two classes: the “Pet” class and the “Dog” class, which inherits from the “Pet” class.

If you have a function for eating called eat() that takes an object of class “Pet” as input and performs the operation of eating on it, then following Liskov’s principle, you should be able to pass an object of class without any problems. Also, use “Dog”.

4. Interface Segregation Principle

The fourth principle of SOLID says that interfaces should be designed so that classes only depend on the part of the interface that is needed, instead of depending on all interfaces.

In general, in programming, an interface is a set of rules and specifications that a class or object must follow to perform its tasks.

Interfaces act as a type of contract between classes. They determine what methods and operations should be available in a class and what variables and properties should be defined in it.

According to the definition of Interface, the fourth principle says that we should design interfaces separately and focus on the requirements of each class. This means that each class only depends on the interfaces it needs to perform its tasks, and the rest of the class does not use these interfaces.

This increases compatibility, reduces unnecessary dependencies, and increases system flexibility.

5. Dependency Inversion Principle

The fifth principle may seem a little more difficult to understand than the previous principles. Simply put, programmers and developers should do their best to minimize dependencies between high-level classes, modules, and objects with low-level modules.

This makes the classes to be used and developed independently of each other. In other words, the goal of the dependency inversion principle is for the programmer to code in such a way that changes in one part of the code have less impact on other parts.

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