\n\nRepost of https://dev.to/rhuzaifa/explained-open-close-principle-in-2-minutes-29dd
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Ever heard about SOLID ?.
\n\n\nSOLID are 5 software development principles or guidelines based on Object-Oriented design making it easier for you to make your projects scalable and maintainable.
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They are more like best practices than rules. Open/Close is a principle of SOLID Design.
\nOpen/Close is the O of SOLID.
\nIt says
\n\n\nSoftware entities (classes, modules, functions, etc.) should be open for extension but closed for modification. Bertrand Meyer
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Like you should not re-write your existing code to be compatible with the newer one.
\n✔️ Can add more code.
\n✔️ Can Extend functionality to old code by adding new code.
\n❌ Can Modify old code used by other modules.
Get the system to a point where you can never break the core of your system. You can add more functionality but not modify the one used by other modules.
\nIt is easier to follow this principle if you already follow the Single Responsibility Principle and it really comes in handy once you also follow the Liskov Substitution Principle.
\nThis prevents those situations in which changing one class also requires you to update all depending classes.
\nWe will use inheritance to demonstrate an example.
\nLet’s say someone is paying for items bought from an E-commerce store. This is what it might look like in payment.ts (ignore the syntax)
class Payment {\n public payWithCash(){\n // handle cash pay logic here\n\n public payWithCredit(){\n \t// handle credit pay logic here\n }\n}\n\nfunction makePayment(payMethod: string) {\n const payment = new Payment();\n\n if(payMethod === 'cash'){\n payment.payWithCash();\n } else if(payMethod === 'credit'){\n payment.payWithCredit();\n }\n}⚠️ What if we want to add a new payment type? We would have to add a new method and modify our code.
\nclass Payment {\n public payWithCash(){\n // handle cash pay logic here\n\n public payWithCredit(){\n // handle credit pay logic here\n }\n\n public payWithCoupon(){\n // handle coupon pay logic here\n }\n}\n\nfunction makePayment(payMethod: string) {\n const payment = new Payment();\n\n if(payMethod === 'cash'){\n payment.payWithCash();\n } else if(payMethod === 'credit'){\n payment.payWithCredit();\n } else if(payMethod === 'coupon'){\n payment.payWithCoupon();\n }\n}This is not in line with the Open/Close Principle.
\nWith the Open/Close Principle we would modify our code payment.ts as follows.
interface PaymentMethod {\n pay() : boolean\n}\n\nclass Cash implements PaymentMethod {\n public pay(){\n // handle cash pay logic here\n }\n}\n\nclass CreditCard implements PaymentMethod {\n public pay(){\n \t// handle credit pay logic here\n }\n}\n\nfunction makePayment(payMethod: PaymentMethod) {\n if(payMethod.pay()){\n return true;\n }\n return false;\n}Now if we, later on, want to add a new payment option like wire transfer then we only need to add a new class, no changes needed to the existing code.
\nclass WireTransfer implements PaymentMethod {\n public pay(){\n // handle wire transfer pay logic here\n }\n}Since we are using the interfaces, be sure to check out if you are following Interface Segregation principle of SOLID.
\nYou should use Open/Close if you want clean code but there are situations in which you should just go ahead and do your own stuff. It really depends on what you are doing.
\nYou can get away with modifying your existing unit tests and just write a new one for the code you added.
\nSo here it is. Do you like the article? Be sure to leave a comment.
\nWhy The Dependency Inversion Principle Is Worth Using.
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