SOLID Principles in JavaScript: Building Robust and Maintainable Applications

As a JavaScript developer, applying the SOLID principles can help ensure a robust, maintainable, and scalable codebase. This article explores what SOLID is and how to apply it to your code.

What are the SOLID Principles?

SOLID is an acronym that stands for five design principles aimed at promoting simpler, more robust, and updatable code for software development in object-oriented languages. The principles were first introduced by Robert C. Martin (also known as „Uncle Bob”) and are widely adopted in the software development industry.

S – Single Responsibility Principle (SRP)

The Single Responsibility Principle states that a class should have only one reason to change. In other words, a class should have a single responsibility or purpose. This principle is essential in JavaScript, where functions and objects are often used to perform multiple tasks.

Example of a class that violates the SRP:

class User {
  constructor(name, email) {
    this.name = name;
    this.email = email;
  }

  save() {
    // Save user data to database
  }

  sendWelcomeEmail() {
    // Send welcome email to user
  }
}

In the example above, the User class has two responsibilities: saving user data to the database and sending welcome emails. This can lead to tight coupling and make the class harder to maintain.

Refactored example that follows the SRP:

class User {
  constructor(name, email) {
    this.name = name;
    this.email = email;
  }

  save() {
    // Save user data to database
  }
}

class EmailService {
  sendWelcomeEmail(user) {
    // Send welcome email to user
  }
}

By separating the concerns of the User class, we’ve made it easier to maintain and test.

O – Open/Closed Principle (OCP)

The Open/Closed Principle states that a class should be open for extension but closed for modification. This means that you should be able to add new functionality to a class without modifying its existing code.

Example of a class that violates the OCP:

class PaymentGateway {
  processPayment(paymentMethod) {
    if (paymentMethod = 'paypal') {
      // Process PayPal payment
    } else if (paymentMethod = 'stripe') {
      // Process Stripe payment
    }
  }
}

In the example above, the PaymentGateway class is not open for extension because adding a new payment method requires modifying the existing code.

Refactored example that follows the OCP:

class PaymentGateway {
  processPayment(paymentMethod) {
    paymentMethod.process();
  }
}

class PayPalPaymentMethod {
  process() {
    // Process PayPal payment
  }
}

class StripePaymentMethod {
  process() {
    // Process Stripe payment
  }
}

By using polymorphism, we’ve made it possible to add new payment methods without modifying the existing code.

L – Liskov Substitution Principle (LSP)

The Liskov Substitution Principle states that subtypes should be substitutable for their base types. This means that any code that uses a base type should be able to work with a subtype without knowing the difference.

Example of a class that violates the LSP:

class Bird {
  fly() {
    // Fly
  }
}

class Penguin extends Bird {
  fly() {
    throw new Error('Penguins cannot fly');
  }
}

In the example above, the Penguin class is not substitutable for the Bird class because it throws an error when trying to fly.

Refactored example that follows the LSP:

class Bird {
  fly() {
    // Fly
  }
}

class FlightlessBird {
  tryToFly() {
    // Try to fly, but fail
  }
}

class Penguin extends FlightlessBird {
  tryToFly() {
    // Try to fly, but fail
  }
}

By creating a separate hierarchy for flightless birds, we’ve made it possible to substitute a Penguin for a FlightlessBird without violating the LSP.

I – Interface Segregation Principle (ISP)

The Interface Segregation Principle states that a client should not be forced to depend on interfaces it does not use. This means that interfaces should be divided into smaller, more focused interfaces that meet the needs of specific clients.

Example of an interface that violates the ISP:

interface Printable {
  print();
  fax();
  scan();
}

class Document implements Printable {
  print() {
    // Print document
  }

  fax() {
    throw new Error('Documents cannot fax');
  }

  scan() {
    throw new Error('Documents cannot scan');
  }
}

In the example above, the Document class is forced to depend on the fax and scan methods of the Printable interface, even though it does not use them.

Refactored example that follows the ISP:

interface Printable {
  print();
}

interface Faxable {
  fax();
}

interface Scannable {
  scan();
}

class Document implements Printable {
  print() {
    // Print document
  }
}

class FaxMachine implements Faxable {
  fax() {
    // Fax document
  }
}

class Scanner implements Scannable {
  scan() {
    // Scan document
  }
}

By dividing the Printable interface into smaller, more focused interfaces, we’ve made it possible for clients to depend only on the interfaces they need.

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. This means that high-level modules should not be tightly coupled to low-level modules, but instead should depend on interfaces or abstract classes.

Example of a class that violates the DIP:

class PaymentProcessor {
  constructor(paymentGateway) {
    this.paymentGateway = paymentGateway;
  }

  processPayment() {
    this.paymentGateway.chargeCard();
  }
}

class PayPalPaymentGateway {
  chargeCard() {
    // Charge card using PayPal
  }
}

In the example above, the PaymentProcessor class is tightly coupled to the PayPalPaymentGateway class.

Refactored example that follows the DIP:

interface PaymentGateway {
  chargeCard();
}

class PaymentProcessor {
  constructor(paymentGateway: PaymentGateway) {
    this.paymentGateway = paymentGateway;
  }

  processPayment() {
    this.paymentGateway.chargeCard();
  }
}

class PayPalPaymentGateway implements PaymentGateway {
  chargeCard() {
    // Charge card using PayPal
  }
}

By depending on the PaymentGateway interface instead of the PayPalPaymentGateway class, we’ve made it possible to swap out the payment gateway without modifying the PaymentProcessor class.

Conclusion

In conclusion, the SOLID principles are a set of guidelines for building robust, maintainable, and scalable software applications. By applying these principles to your JavaScript code, you can ensure that your applications are easy to modify, extend, and test.