In software development, especially in object oriented programming, the SOLID principles help create systems that are easy to maintain, extend, and test.

S — Single Responsibility Principle (SRP)

→ This principle states that a class or module should have only one responsibility.

E-Commerce example:

Think of an order.

Instead of one big class that:

• creates orders

• calculates prices

• sends emails

• stores orders in a database

We split it.

Good example:

class OrderCreator {}
class PriceCalculator {}
class EmailNotifier {}
class OrderRepository {}

So if tomorrow the email format changes, you only update EmailNotifier and nothing else breaks.

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O — Open Closed Principle (OCP)

→ You should add features without changing old code.

E-Commerce example:

You already support Cash on Delivery and Credit Card payments.
Now you want to add UPI or PayPal.

Bad approach is editing old code and adding more if else.

Good approach is allow adding new payment classes.

class Payment {}
class CashOnDelivery extends Payment {}
class CreditCardPayment extends Payment {}
class PayPalPayment extends Payment {}

The app works without rewriting old payment code.

So adding new features is safe and clean.

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L — Liskov Substitution Principle (LSP)

→ Child classes should behave like parent classes. (LSP states that child classes should be replaceable for their parent classes without changing the behavior of the system)

E-Commerce example:

Suppose you have a base Payment class.
Any new payment like Stripe or Razorpay must follow the expected behavior.

If parent says:

payment.pay()

Then child classes must implement pay() properly.

A child should not break the rules by:

• throwing errors

• changing return types

• not processing payment

So anywhere Payment is used, StripePayment must work the same way.

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I — Interface Segregation Principle (ISP)

→ Do not force classes to use unnecessary methods. ISP focuses on creating small and specific interfaces rather than large and general ones.

E-Commerce example:

Imagine one big interface:

class EcommerceService {
  pay() {}
  shipOrder() {}
  trackOrder() {}
  applyDiscount() {}
}

A payment class should not need shipOrder() or trackOrder().

Better approach:

class PaymentService {}
class ShippingService {}
class DiscountService {}

Each module does only what it needs.

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D — Dependency Inversion Principle (DIP)

Meaning: High level modules should depend on abstractions, not fixed implementations.

E-Commerce example:

Bad approach:

class Order {
  constructor() {
    this.payment = new PayPal();
  }
}

Order depends on PayPal only.

If PayPal fails or you change provider, you change Order.

Good approach:

class Order {
  constructor(paymentGateway) {
    this.payment = paymentGateway;
  }
}

Now you can pass:

new PayPal()
new Stripe()
new Razorpay()

without changing the Order class.

This makes your code flexible and testable.

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Why SOLID matters

• reduce bugs when updating code

• support easier refactoring

• enable more flexible systems

• write cleaner unit tests

• avoid tight coupling across modules

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When to use SOLID

• large codebases

• long term projects

• systems expecting future extensions

• object oriented designs

It can be less important for small scripts or prototypes, but understanding it still builds good habits.

Thank You