What is an interface?

Learn to define object shapes with Interface.

What is an interface?

An interface defines the shape of an object: which properties it has and their types. It acts as a contract that objects must follow. It is defined with the interface keyword.

interface Kisi {
  ad: string;
  yas: number;
}
let kisi: Kisi = { ad: "Ali", yas: 25 };

An interface checks object structure at compile time.

An interface is a contract

When you use an interface as a type, the object must match the interface exactly. Missing or extra properties cause compile errors. This creates a contract between different parts of the code.

interface Kisi {
  ad: string;
  yas: number;
}
let kisi: Kisi = {
  ad: "Ali",
  yas: 25
};  // valid

An interface guarantees the object follows the contract.

Exercise Define and use an interface

Define an interface named Kisi with properties ad: string and yas: number. Declare a variable named kisi of type Kisi with value { ad: "Ayşe", yas: 28 }. Print the value of kisi.ad with console.log.

Show example solution

interface Kisi {
    ad: string;
    yas: number;
}
let kisi: Kisi = { ad: "Ayşe", yas: 28 };
console.log(kisi.ad);

Test yourself

What does an interface define?
- A variable's value
- An array's length
- The shape of an object
- A function's return type
Show answer

The shape of an object — An interface specifies an object's shape by defining property names and types; it is a contract that objects must follow.

Complete the interface property

interface Nokta {
  x: number;
  y: ____;
}

Show answer

interface Nokta {
  x: number;
  y: number;
}

Interface concepts

interface Kisi { ... }

Object shape definition
interface

Contract definition
let k: Kisi = { ... }

Using interface as a type
Interface

Specifying object structure

Next lesson: Property types

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies