Property types

Learn to type interface properties.

Property types

Each property in an interface has a type. You can use string, number, boolean, arrays, or even other interfaces. Properties are separated by semicolons.

interface Urun {
  ad: string;
  fiyat: number;
  stokta: boolean;
}

Each property determines the type of the object's corresponding value.

Optional and readonly in interfaces

Interfaces support optional (?) and readonly properties just like object types. Optional properties may be absent; readonly properties cannot be changed after creation.

interface Urun {
  readonly id: number;
  ad: string;
  etiketler?: string[];
}

An interface supports all features of object types.

Exercise Interface with various property types

Define an interface named Urun with properties ad: string, fiyat: number, and stokta: boolean. Declare a variable named urun of type Urun with value { ad: "kalem", fiyat: 15, stokta: true }. Print the value of urun.fiyat with console.log.

Show example solution

interface Urun {
    ad: string;
    fiyat: number;
    stokta: boolean;
}
let urun: Urun = { ad: "kalem", fiyat: 15, stokta: true };
console.log(urun.fiyat);

Test yourself

In <code>interface Urun { ad: string; fiyat: number; stokta: boolean }</code>, what is the type of the <code>fiyat</code> property?
- Urun
- string
- boolean
- number
Show answer

number — Since the interface definition writes <code>fiyat: number</code>, the type of the <code>fiyat</code> property is <code>number</code>.

Complete the property type

interface Urun {
  ad: string;
  fiyat: number;
  stokta: ____;
}

Show answer

interface Urun {
  ad: string;
  fiyat: number;
  stokta: boolean;
}

Property types concepts

ad: string (interface)

String property
fiyat: number (interface)

Number property
etiketler?: string[]

Optional array property
readonly id: number

Readonly property

Next lesson: Interface vs type alias

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