Readonly properties

Learn immutable properties with readonly.

Readonly properties

To make object properties immutable, you write readonly before the property name. Once set, a readonly property cannot be changed.

let kisi: { readonly id: number; ad: string } = {
  id: 1,
  ad: "Ali"
};

readonly prevents the property from being modified later.

Using readonly properties

Readonly properties are suitable for values that should never change, such as identifiers or codes. Attempting to assign to a readonly property causes a compile error.

let kisi: { readonly id: number } = { id: 1 };
kisi.id = 2;  // error: readonly

Readonly prevents accidental modification at compile time.

Exercise Object with a readonly property

Declare a variable named urun with type { readonly kod: string; fiyat: number } and value { kod: "A100", fiyat: 50 }. Print the value of urun.kod with console.log.

Show example solution

let urun: { readonly kod: string; fiyat: number } = { kod: "A100", fiyat: 50 };
console.log(urun.kod);

Test yourself

What does <code>readonly id: number</code> indicate?
- id property is of type string
- id property can be any type
- id property is optional
- id property cannot be changed after creation
Show answer

id property cannot be changed after creation — <code>readonly</code> indicates that the property cannot be changed after the first assignment; <code>id</code> cannot be reassigned once created.

Make the property readonly

let kullanici: { ____ id: number; ad: string } = { id: 1, ad: "Ali" };

Show answer

let kullanici: { readonly id: number; ad: string } = { id: 1, ad: "Ali" };

Readonly property concepts

readonly id: number

Immutable id property
id: number (readonly yok)

Mutable id property
readonly (özellik önünde)

Makes a property fixed
Readonly property

Cannot be reassigned later

Next lesson: Summary - Objects

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