CLEAN CODE and S.O.L.I.D structure with nodejs and typescript

SOLID is an Acronym created by Michael Feathers to describe the five design patters most discussed these days

• S: Single-responsibility principle
• O: Open–closed principle
• L: Liskov substitution principle
• I: Interface segregation principle
• D: Dependency inversion principle

Single-responsibility principle: A class should only have a single responsibility, that is, only changes to one part of the software's specification

should be able to affect the specification of the class. Ps: the SRP is also applicable for functions and methods.

Example:

class UserRepository {
  createUserIfNotExists(user: User) {}
}
class CreateUserController {
  exec(user: User) {}
}
class UserScore {
  score(user: User) {}
}

Open–closed principle: "Software entities should be open for extension, but closed for modification."

Example:

interface PaymentMethod {}
class CreditCard implements PaymentMethod {}
class Boleto implements PaymentMethod {}

Liskov substitution principle: "Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program."

Examples of principle violation:

• Implements a method that does nothing
• Throw a unexpected exception
• Return different values

Example:

class PaymentMethod {}
class creditCard extends PaymentMethod {}
class Checkout {
  constructor(payment: PaymentMethod) {}
}
const creditCard = new CreditCard();
const checkout = new Checkout(creditCard); //No Errors

Interface segregation principle: "Many client-specific interfaces are better than one general-purpose interface."

Example:

class PaymentMethod {}

class ElectronicPayment extends PaymentMethod {}
class CashPayment extends PaymentMethod {}

class CreditCard extends EletronicPayment {}
class Boleto extends CashPayment {}

This prevents the "Boleto" class from unnecessarily implementing things like: interest or installments

Dependency inversion principle: One should "depend upon abstractions, not concretions."

class MasterCard implements IPaymentMethod {}
class Checkout {
  constructor(paymentMethod: IPaymentMethod) {}
}

const creditCard = new MasterCard();
const checkout = new Checkout(creditCard);