CodeBlanks Logo Icon CodeBlanks
Login Register

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.

Practice 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.

Starter code

index.ts
// urun adında bir nesne tanımla
// kod özelliği readonly dize, fiyat özelliği sayı olsun
// kod değeri "A100", fiyat değeri 50 olsun
// urun.kod değerini yazdır

Sample solution

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

Expected output

A100

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 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 Correct answer

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

2 Make the property readonly

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

Explanation: To make a property immutable, the <code>readonly</code> keyword is written before the property name; <code>let</code>, <code>const</code> and <code>static</code> are not used as property modifiers.

3 Readonly property concepts

Term Meaning
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

All lessons in Beginner

1 TypeScript intro and setup

  1. What is TypeScript?
  2. Why TypeScript?
  3. tsc compiler
  4. tsconfig basics
  5. Summary - TypeScript basics

2 Primitive types

  1. string type
  2. number type
  3. boolean type
  4. let and const
  5. Summary - Primitive types

3 Arrays

  1. Array type
  2. Array methods
  3. readonly arrays
  4. Multidimensional arrays
  5. Summary - Arrays

4 Tuples

  1. What is a tuple?
  2. Tuple usage
  3. readonly tuple
  4. Summary - Tuple

5 Enums

  1. Numeric enum
  2. String enum
  3. const enum
  4. Enum members
  5. Summary - Enum

6 any, unknown, never

  1. any type
  2. unknown type
  3. never type
  4. void type
  5. Summary - Special types

7 Type annotation

  1. Variable annotation
  2. Parameter annotation
  3. Return type annotation
  4. Summary - Type annotation

8 Type inference

  1. Basic inference
  2. Inference with const
  3. Best common type
  4. Summary - Type inference

9 Objects

  1. Object type
  2. Optional properties
  3. Readonly properties
  4. Summary - Objects

10 Functions

  1. Function declaration
  2. Optional and default params
  3. Rest params
  4. Function overloads
  5. Summary - Functions

11 Arrow functions

  1. Arrow syntax
  2. this with arrow
  3. Arrow type
  4. Summary - Arrow functions

12 Interfaces

  1. What is an interface?
  2. Property types
  3. Interface vs type alias
  4. Summary - Interfaces

13 Type alias

  1. What is a type alias?
  2. Composition with alias
  3. Alias vs interface
  4. Summary - Type alias

14 Union and intersection

  1. Union types
  2. Intersection types
  3. Literal types
  4. Summary - Union & intersection

15 null and undefined

  1. null vs undefined
  2. Optional chaining
  3. Nullish coalescing
  4. Summary - null & undefined

16 Strict mode

  1. strict: true
  2. strictNullChecks
  3. strictFunctionTypes
  4. Summary - Strict mode

17 Project: Shopping cart

  1. Cart design
  2. Add product
  3. Total calculation
  4. Apply discount
  5. Integration

18 Project: Todo CLI

  1. Task model
  2. Add and list
  3. Completion
  4. Filtering
  5. Integration

19 Project: Library

  1. Book model
  2. Add and search
  3. Borrow
  4. Return
  5. Integration

20 Beginner final

  1. Challenge 1: Types
  2. Challenge 2: Functions
  3. Challenge 3: Unions
  4. Challenge 4: Mini app