TypeScript offers powerful way for organizing code and managing relationships between different components through the use of extends and implements keywords. This article explores the distinctions and functionalities of these two keywords.

Extends

The extends keyword is used for two main purposes in TypeScript: inheriting from a class (class inheritance) and extending a type (type extension).

Class Inheritance

When using extends for class inheritance, a subclass is created that inherits properties and methods from another class.

Syntax (For class inheritance):

class DerivedClass extends BaseClass {
  // Additional properties and methods in the derived class
}

Syntax (For type extension):

type ExtendedType = BaseType & {
  // Additional properties and methods in the extended type
};

Example 1: In this example, we are using extends for class inheritance

class Animal {
    makeSound(): void {
        console.log('Dog sound');
    }
}
// Derived class representing a 
// specific type of animal: Dog
class Dog extends Animal {
    bark(): void {
        console.log('Dog is barking!');
    }
}

const myDog = new Dog();
myDog.makeSound();
myDog.bark();      

Output:

Dog sound
Dog is barking!

Example 2: In this example, we are using extends for type extension.

type Shape = {
    color: string;
};

type Square = Shape & {
    sideLength: number;
};
const mySquare: Square = {
    color: 'red',
    sideLength: 5,
};
console.log(mySquare);

Output:

{ color: 'red', sideLength: 5 }

Implements

The implements keyword is used to ensure that a class conforms to a particular interface. This enforces that the class implements all the properties and methods defined by the interface.

Syntax:

class ClassName implements InterfaceName {
  // Class properties and methods
}

Example: In this example, the GFG class implements the GeeksForGeeks interface, and it is required to provide an implementation for the print method specified by the interface.

interface GeeksforGeeks {
    print(): void;
}

class GFG implements GeeksforGeeks {
    print() {
        console.log('GeeksForGeeks');
    }
}
const output = new GFG();
output.print();  

Output:

GeeksForGeeks

Difference between extends and implements