Interface Segregation Principle In Typescript
Introduction
Interface Segregation Principle
is the 4th principle of SOLID
principles. It is also known as ISP
principle. It was first introduced by Robert C. Martin
in his 2000 paper Design Principles and Design Patterns
.
Definition
The concept is simple, make the interfaces smaller and more specific to the use case. it is better to have multiple but smaller interfaces than a single large interface.
So basically, a class should not be forced to implement an interface or methods that it does not use.
Key Concepts Of ISP
-
Cohesion
: Interfaces should be highly cohesive. It means the interface methods should be related to each other, if they are not related then they should be separated into different interfaces. -
Responsibility
: Interfaces should have single responsibility.
Example
Let's take an example of a Printer
class that can print, scan and fax. So we can create an interface MultiFunctionPrinter
that has all the methods of Printer
class.
interface MultiFunctionPrinter {
print(): void;
scan(): void;
fax(): void;
}
Now we have a SimplePrinter
class that can only print. So we can implement the MultiFunctionPrinter
interface in SimplePrinter
class.
class SimplePrinter implements MultiFunctionPrinter {
print(): void {
console.log("Printing...");
}
scan(): void {
throw new Error("Simple Printer can not scan.");
}
fax(): void {
throw new Error("Simple Printer can not fax.");
}
}
This is a total violation of ISP
principle. Because SimplePrinter
class is forced to implement the scan
and fax
methods that it does not use.
Solution
To make this run smoothly, we can separate the interface into three different interfaces.
interface Printer {
print(): void;
}
interface Scanner {
scan(): void;
}
interface Fax {
fax(): void;
}
Now we can implement the interfaces in SimplePrinter
class.
class SimplePrinter implements Printer {
print(): void {
console.log("Printing...");
}
}
If there is advanced printer class, it can implement multiple interfaces.
class AdvancedPrinter implements Printer, Scanner, Fax {
print(): void {
console.log("Printing...");
}
scan(): void {
console.log("Scanning...");
}
fax(): void {
console.log("Faxing...");
}
}
Conclusion
ISP
principle is very important to make the code more maintainable and flexible. It also helps to reduce the coupling between classes and not implementing unnecessary methods.
Following the list
You can see the updated list of design principles from the following link
https://mentoor.io/en/posts/634524154/open-closed-principle-in-typescript
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