implements in TypeScript: Enforcing Interface Contracts

In TypeScript, the implements keyword is used to force a class to conform to an interface.

This means the class must implement all properties and methods described in the interface. Otherwise TypeScript will throw a compilation error.

Syntax

interface IAnimal {
  name: string;
  makeSound(): void;
}

class Dog implements IAnimal {
  name: string;

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

  makeSound() {
    console.log("Woof!");
  }
}

What happens?

• Class Dog must implement all properties and methods of interface IAnimal.
• If at least one is missing — TypeScript will show an error.

Error Example

interface Person {
  name: string;
  age: number;
}

class User implements Person {
  name: string;
  // Error: Property 'age' is missing
}

implements with multiple interfaces

Can implement multiple interfaces separated by commas:

interface A {
  a(): void;
}

interface B {
  b(): void;
}

class C implements A, B {
  a() {
    console.log("A");
  }

  b() {
    console.log("B");
  }
}

Why use implements?

• For structure control of class
• For API documentation: it's clear what class should support
• For architectural compliance guarantees
• For modularity and reusability: one interface — many implementations