Elevate Your JavaScript Game With SOLID Principles: Best Practices For Clean And Scalable Code!




May 30, 2023

Last updated: January 3, 2024

solid principles in JavaScript

JavaScript: The Swiss Army Knife Of Web Development

JavaScript is a versatile and popular programming language used by numerous web developers worldwide. Its lightweight and easy-to-learn syntax makes it a great choice for creating interactive web pages, in web application development, and even server-side scripting. JavaScript can be used to manipulate web page content, create animations, validate form data, and much more.

However, in JavaScript, as the projects become more complex, it can be challenging to maintain and scale the codebase. This is exactly where SOLID principles in JavaScript come into place – a set of five object-oriented design principles that can help you write more maintainable and scalable code. Thus in this blog, let’s unpack each of these principles and look further into how they can be applied to JavaScript.

What Is SOLID Principles In JavaScript?

What exactly is SOLID Principles in JavaScript? The SOLID principles in JavaScript are a set of design principles that aim at improving the quality and maintainability of the software. Although originally formulated for object-oriented programming, many of these principles can efficiently be applied to JavaScript as well.

The SOLID Principles In JavaScript: Code With Utmost Clarity

The SOLID JavaScript principles provide developers with a set of guidelines to write software that is modular, easy to understand, and easy to modify. By adhering to these SOLID principles, you can create a software that is more flexible, maintainable, and scalable. 

But before we jump into the in-depth analysis of SOLID principles, let us first understand what “SOLID” stands for:


S — Single Responsibility Principle

O — Open-Closed Principle

L — Liskov Substitution Principle

I — Interface Segregation Principle

D — Dependency Inversion Principle

solid principles

S — Single Responsibility Principle (SRP):

The Single Responsibility Principle emphasizes that each class or module should have only one responsibility, meaning that it should only have one reason to change. In JavaScript, this can be achieved by breaking down complex functions into smaller ones that perform a single task.

For example, instead of having a function that handles both user authentication and database connection, you can separate them into two separate functions, each with its own responsibility.

single responsibility principle

An illustration of single responsibility principle example

The above single responsibility principle example illustrates how the SRP principle is applied differently. The function authenticateUser manages both user authentication and database connection in the flawed example. Here, the Single Responsibility Principle, which asserts that a function should only have one responsibility, is broken by this inferior practice.

Nonetheless, it becomes more challenging to reuse the code when the authentication and database connection logic are combined into a single function. This is because the database connection logic may not be required in other areas of the code. The fact that any modifications to the authentication or database connection logic must be made in the same function makes it more difficult to maintain and debug the code.

AuthenticateUser and connectToDatabase are two distinct functions in the above example that separates the logic for authentication and database connection. The connectToDatabase method may be used in other portions of the code and the modifications made to one function won’t affect the other. Thus this enables the code to be more modular and reusable.

Here the code is simpler to test because each of the functions can be tested individually when the concerns are separated. Nonetheless, since the defects can be found and addressed more readily, the quality and dependability of the code can be seamlessly enhanced. 

Pros Of The Single Responsibility Principle:

  • Easier to maintain and modify the code
  • Enhanced modular code
  • Easier to read and test the code
  • Less prone to errors

Cons Of Single Responsibility Principle:

  • Increases the number of functions or modules in the codebase
  • Can require more time to implement

Also Read: Comprehensive Guide To Garbage Collection In JavaScript

O — Open Closed Principle (OCP):

The Open Closed Principle states that the classes or modules should be open for an extension but closed for modification. This means that you should be able to add new functionality to a class or module without modifying its existing code. In JavaScript, this can be achieved by using interfaces, abstract classes, and inheritance.

Open closed principle

The logData() function in the first open closed principle example determines whether the data supplied to it is an instance of the Error class and calls the console.error() if it is, or console.log() if it is not. However, if there are many different types of data to manage, it can become cumbersome and might not be extendable if additional data types are added at a later stage.  

The second open closed principle example uses an abstract class Logger that includes a log() function and offers a more adaptable approach. Different subclasses that extend the Logger class can each implement the log() method differently for particular types of data. In this instance, the ConsoleLogger subclass uses console.log() to log data to the console, but the ErrorLogger subclass uses console.error to log Error objects.().

Thus an abstract class and its subclasses provide a more modular and extensible approach that makes it simple to add new data types in the future without changing the present code to use.  

Pros Of Open-closed principle:

  • More flexible code
  • Easier to modify and extend code
  • Less prone to errors

Cons Of Open-closed principle:

  • Can require more time to implement
  • Can increase the complexity of the codebase

L – Liskov Substitution Principle (LSP):

The Liskov Substitution Principle states that the objects of a superclass should be in a position to be replaced with objects of a subclass without affecting the correctness of the program. In JavaScript, this can be achieved by ensuring that the subclasses inherit the same properties and methods as their parent classes.

Liskov substitution principle

In the above example, the base class is Developer, and we have two subclasses: FrontendDeveloper and BackendDeveloper, each with their specific implementation of the code method.

Additionally, we introduce the CalibraintDeveloper class, which extends the Developer class and overrides the code method with a marketing message related to Calibraint. The marketing message emphasizes that the Calibraint developers create innovative solutions.

The makeDeveloperCode function accepts a Developer object and calls its code method. By using the Liskov Substitution Principle, we can pass any subclass of Developer to the function without breaking the expected behavior.

The example showcases how the Liskov Substitution Principle can be applied while incorporating a marketing message related to Calibraint.

Pros Of Liskov Substitution Principle:

  • More predictable code
  • Easier to test code
  • Less prone to errors

Cons Of Liskov Substitution Principle:

  • Can increase the complexity of the codebase
  • Might require more time to implement

I – Interface Segregation Principle (ISP):

The Interface Segregation Principle states that clients should not be forced to depend on interfaces that they do not use. In JavaScript, this can be achieved by creating smaller interfaces with fewer methods instead of larger interfaces with numerous methods.

interface segregation principle

An illustration of interface segregation principle example

The first interface segregation principle example demonstrates a poor implementation of the Calibraint class, which has a broad interface and numerous methods like developer(), tester(), and designer(). In other words, even if it does not make any sense for that specific class to implement all of these methods, it would be required for any class that implements the Calibraint interface. This reduces the code’s flexibility & maintainability and adds pointless dependencies between classes.

The second example demonstrates a better implementation of the Calibraint class, which has been divided into three more manageable interfaces with fewer methods each: CalibraintDesigner, CalibraintTester, and CalibraintDesigner. According to the ISP, classes should only be made dependent on the methods that they actually need and shouldn’t be made to depend on extraneous methods.

Here, the interface is divided into smaller & more focused interfaces to provide flexibility and lets the classes just implement the methods that they actually require. With this method, the code is simpler to maintain, more versatile, and better suited to meet changing requirements.

Pros Of Interface Segregation Principle:

  • More modular code
  • Easier to maintain and modify code
  • Less prone to errors

Cons Of Interface Segregation Principle:

  • Can increase the number of interfaces in the codebase
  • Can require more time to implement

D- Dependency Inversion Principle (DIP):

The Dependency Inversion Principle states that high-level modules should not depend on low-level modules, but both should depend on abstractions. In JavaScript, this can be achieved by using dependency injection, where dependencies are passed to a function or class instead of being created inside it. Having said that, let us see a dependency inversion principle example.

dependency inversion principle

The Dependency Inversion Principle example of the SOLID principles for object-oriented programming is shown above.

The ShoppingCart class in the poor example is directly dependent on the Cart class. As a result, the two classes become closely connected, making it possible for modifications to one class to necessitate changes in the other.

A generalization of the Cart class, which is provided as a function Object() { [native code] } argument in a good example is what the ShoppingCart class depends on. Nonetheless, greater flexibility is made possible since several Cart class implementations can be utilized without having an impact on the ShoppingCart class.

Pros Of Dependency Inversion Principle:

  • Promotes loose coupling between modules.
  • Increases code flexibility and maintainability.
  • Easier to change or modify lower-level modules without affecting higher-level modules.
  • Promotes unit testing and improves testability.

Cons Of Dependency Inversion Principle:

  • Can be time-consuming to implement in larger projects.
  • May require additional interfaces or abstraction layers, which can increase the complexity.

In Retrospect

JavaScript is a highly versatile programming language that supports multiple paradigms, including object-oriented, functional, and procedural programming. By applying SOLID principles in JavaScript code, you can significantly enhance the readability, maintainability, and scalability of your code. One of the great benefits of JavaScript is that it helps to combine SOLID JavaScript principles with functional programming techniques, which allows us to write code that is both clean and efficient. However, it’s important to note that there are many ways to achieve SOLID principles in JavaScript, and what we’ve presented is just one approach. As a dynamic programming language, JavaScript & the SOLID principles in JavaScript provides developers with the flexibility and power to create complex applications that can adapt and evolve over time.

However, writing clean code is not only about making it look pretty but also about making it easy to understand and modify over time. While these principles may seem a bit abstract at first, they can remarkably help to improve the quality of code and reduce the time & effort required to maintain it. By following these principles, developers can create code that is easy to understand, test, and modify, which ultimately leads to better custom software development. Nonetheless, with the rise of web app development frameworks such as React and Node.js, JavaScript will continue to evolve and remain a fundamental tool for modern web app development. 

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