Implements and Extends, Object Oriented TypeScript

TypeScript uses implements and extends as keywords in class declarations. They are both ‘similar’, but fundamentally different. I’ve recently had developers get confused over the two so I thought I would put up a post for reference.

Note: To keep the tutorial simple we won’t be covering extending interfaces or mixing extends and implements.

When to use Implements

We use implements for implementing interfaces. Interfaces are like contracts and define a set of rules that an object must follow. If a class implements an interface then it has to implement all of the properties and methods defined in the interface.

interface GavsInterface {
    name: string;
    speak(): void;
}

class GavsClass implements GavsInterface {
    name: string;

    constructor(name: string) {
        this.name = name;
    }

    speak(): void {
        console.log(this.name);
    }
}

As you can see, my class is implementing my interface. This means that my class must provide an implementation for both the name property and speak() method.

Note: You can also use implements when interfacing from a Type. A Type is quite similar to an interface, but a lot more general. Types can also define primitive types, unions and intersections.

type BaseType = {
    name: string
}

class GavsClass implements BaseType {
    name: string;

    constructor(name: string) {
        this.name = name;
    }
}

When to use Extends

Extends is a bit different and is used to inherit properties and methods directly from another class. This means that when we extend a class, the new class will inherit all of the properties and methods from the base class. The new class can then add more of its own properties and methods.

class BaseClass {
    name: string = "Gav";

    speak(): void {
        console.log(this.name);
    }
}

class GavsClass extends BaseClass {
    age: number = 37;
    
    bio(): void {
        console.log(`${this.name} is ${this.age} years old`);
    }
}

Here, GavsClass extends the BaseClass and therefore inherits the properties and methods from BaseClass. We then implement the age property and bio() method in GavsClass. The bio() method is able to use the properties from the BaseClass because GavsClass has inherited the properties and methods.

We can also extend from abstract classes. Abstract classes are classes which can’t be instantiated directly. They are blueprints for other classes with both abstract and concrete methods.

abstract class AbstractClass {
    protected name: string;

    constructor(name: string) {
        this.name = name;
    }

    speak(): void {
        console.log(this.name);
    }
    
    abstract bio(): void;
}

class GavsClass extends AbstractClass {
    age: number;

    constructor(name: string, age: number) {
        super(name);

        this.age = number;
    }
    
    bio(): void {
        console.log(`${this.name} is ${this.age} years old`);
    }
}

With this example we can see some differences. We are declaring the bio() method as abstract, with no implementation. This means that GavsClass has to implement the method when it inherits from AbstractClass.

Note: In this example we are also using super() to call the base constructor. This is allowing us to set both name and age in the constructor of GavsClass.

You should use implements when you want to specify a set of rules, or ‘contract’, that a class must follow, and use extends when you want to create a subclass that inherits from a base class.