implements vs extends | Engineering Interview Kit

TL;DR

extends (Inheritance): A class copies all properties and methods from a base class. It establishes an “Is-A” relationship.
implements (Contract): A class agrees to adhere to the shape defined by an interface. It establishes a “Can-Do” relationship.

Mental Model

How It Works

`extends`

When a class extends another class, it inherits its actual runtime code. If the base class has a greet() method, the child class automatically gets it without writing any new code. You can only extend one class at a time (JavaScript does not support multiple inheritance).

`implements`

When a class implements an interface, it inherits absolutely no code. The interface acts as a strict checklist. The class must manually write out all the properties and methods listed in the interface. You can implement multiple interfaces at the same time (separated by commas).

Example

// --- The Class to EXTEND ---
class BaseEntity {
    id: string = Math.random().toString();
    save() { console.log("Saved to DB!"); }
}

// --- The Interfaces to IMPLEMENT ---
interface Pingable {
    ping(): void;
}
interface Loggable {
    log(msg: string): void;
}

// --- The Subclass ---
// Extends exactly 1 class, Implements N interfaces
class Server extends BaseEntity implements Pingable, Loggable {
    // We get 'id' and 'save()' for free from BaseEntity!

    // We MUST write the implementation for ping()
    ping() {
        console.log("Pong");
    }

    // We MUST write the implementation for log()
    log(msg: string) {
        console.log(`[Server ${this.id}]: ${msg}`);
    }
}

Common Interview Questions

Can an interface `extend` a class?

Surprisingly, Yes! In TypeScript, an interface can extend a class. When it does, it inherits all the class’s property types and method signatures (including private and protected ones), but not their implementations. This is an advanced feature usually avoided because it tightly couples the interface to the class hierarchy.

What happens if you try to `implement` an interface, but you add extra properties to the class?

That is perfectly fine. implements only guarantees that the minimum requirements of the interface are met. The class is free to add as many extra properties or methods as it wants.

TL;DR

Mental Model

How It Works

extends

implements