What is an interface?
- TypeScript course
- Beginner
- Lesson 51/89
- 4 min
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
}; // validAn interface guarantees the object follows the contract.
Practice 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.
Starter code
// Kisi adında bir interface tanımla
// ad (string) ve yas (number) özellikleri içersin
// kisi değişkenini Kisi türünde tanımla, değer: { ad: "Ayşe", yas: 28 }
// kisi.ad değerini yazdırSample solution
interface Kisi {
ad: string;
yas: number;
}
let kisi: Kisi = { ad: "Ayşe", yas: 28 };
console.log(kisi.ad);Expected output
Ayşe
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does an interface define?
- A variable's value
- An array's length
- The shape of an object Correct answer
- A function's return type
Explanation: An interface specifies an object's shape by defining property names and types; it is a contract that objects must follow.
2 Complete the interface property
interface Nokta {
x: number;
y: ____;
}interface Nokta {
x: number;
y: number;
}Explanation: A point's <code>y</code> coordinate is a number; <code>number</code> is the correct type. <code>string</code> and <code>boolean</code> are wrong for a coordinate, and <code>void</code> is not a property type.
3 Interface concepts
| Term | Meaning |
|---|---|
interface Kisi { ... } |
Object shape definition |
interface |
Contract definition |
let k: Kisi = { ... } |
Using interface as a type |
Interface |
Specifying object structure |