Readonly utility type in Typescript

Introduction

The Readonly utility type helps you make an object's properties unchangeable. This means you can protect data from being accidentally changed.

When you want to make sure an object's data stays the same after creation.

When sharing data between parts of a program and you want to avoid accidental changes.

When working with configuration objects that should not be modified.

When you want to clearly show that some data is meant to be constant.

When you want to catch errors early by preventing property changes in your code.

Syntax

Typescript

Readonly<Type>

Type is the object type you want to make read-only.

The result is a new type where all properties are readonly.

Examples

This creates a user object whose name and age cannot be changed.

Typescript

type User = {
  name: string;
  age: number;
};

const user: Readonly<User> = {
  name: "Alice",
  age: 30
};

Trying to change point.x will cause a TypeScript error because it is readonly.

Typescript

type Point = {
  x: number;
  y: number;
};

const point: Readonly<Point> = { x: 10, y: 20 };

// point.x = 15; // Error: Cannot assign to 'x' because it is a read-only property.

Sample Program

This program creates a readonly myBook object and prints its title. Trying to change the title will cause an error.

Typescript

type Book = {
  title: string;
  author: string;
};

const myBook: Readonly<Book> = {
  title: "Learn TypeScript",
  author: "Jane Doe"
};

console.log(myBook.title);

// Uncommenting the next line will cause a TypeScript error
// myBook.title = "New Title";

Important Notes

Readonly only affects the top-level properties. Nested objects inside are not automatically readonly.

Use Readonly to help prevent bugs by making data immutable.

Summary

Readonly makes all properties of an object type unchangeable.

It helps protect data from accidental changes.

It is useful for constants, configs, and safer code.

Practice

1. What does the Readonly utility type do in TypeScript?

easy

A. Makes all properties optional

B. Allows properties to be changed only once

C. Removes all properties from an object type

D. Makes all properties of an object type unchangeable

Solution

1. Step 1: Understand the purpose of Readonly

   The Readonly utility type is designed to make all properties of an object type immutable, meaning they cannot be changed after initialization.

2. Step 2: Compare with other options

   Options B, C, and D describe behaviors that are not related to Readonly. Only Makes all properties of an object type unchangeable correctly describes making properties unchangeable.

3. Final Answer:

   Makes all properties of an object type unchangeable -> Option D

4. Quick Check:

   Readonly = makes properties unchangeable [OK]

Hint: Readonly means no changes allowed to object properties [OK]

Common Mistakes:

• Thinking Readonly allows one-time changes
• Confusing Readonly with Partial or Required types
• Believing Readonly removes properties

2. Which of the following is the correct syntax to create a readonly type from an interface User?

easy

A. type ReadonlyUser = Readonly<User>();

B. type ReadonlyUser = Readonly<User>;

C. type ReadonlyUser = readonly User;

D. type ReadonlyUser = Readonly<User>[];

Solution

1. Step 1: Recall the syntax of Readonly utility

   The correct syntax to create a readonly version of a type or interface is Readonly<TypeName> without parentheses or array brackets.

2. Step 2: Analyze each option

   type ReadonlyUser = Readonly<User>; uses the correct generic syntax. type ReadonlyUser = Readonly<User>(); incorrectly uses parentheses. type ReadonlyUser = readonly User; uses lowercase 'readonly' which is a keyword for arrays or tuples, not for types. type ReadonlyUser = Readonly<User>[]; creates an array of readonly users, which is different.

3. Final Answer:

   type ReadonlyUser = Readonly<User>; -> Option B

4. Quick Check:

   Readonly syntax = Readonly<Type> [OK]

Hint: Use Readonly<Type> without parentheses or brackets [OK]

Common Mistakes:

• Adding parentheses after Readonly
• Using lowercase 'readonly' instead of utility type
• Confusing array of readonly with readonly type

3. What will be the output of the following TypeScript code?

interface Config {
  url: string;
  timeout: number;
}

const config: Readonly<Config> = { url: 'api.com', timeout: 5000 };

config.timeout = 3000;
console.log(config.timeout);

medium

A. Compilation error

B. 5000

C. 3000

D. undefined

Solution

1. Step 1: Understand Readonly effect on assignment

   The Readonly type makes all properties immutable. Trying to assign a new value to config.timeout will cause a compilation error.

2. Step 2: Check the code behavior

   The line config.timeout = 3000; is invalid and TypeScript will not compile this code, so no runtime output occurs.

3. Final Answer:

   Compilation error -> Option A

4. Quick Check:

   Readonly blocks property assignment = Compilation error [OK]

Hint: Readonly properties cannot be assigned new values [OK]

Common Mistakes:

• Assuming readonly allows property changes
• Expecting old value printed despite error
• Confusing runtime error with compile error

4. Identify the error in this TypeScript code using Readonly and fix it:

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

const p: Readonly<Point> = { x: 10, y: 20 };
p.x = 15;
console.log(p.x);

medium

A. Change p.x = 15; to p = { x: 15, y: 20 };

B. Use as const instead of Readonly

C. Remove Readonly to allow property changes

D. Declare p as let instead of const

Solution

1. Step 1: Identify why assignment fails

   The Readonly type prevents changing properties like p.x</>. The error is caused by trying to assign p.x = 15;.

2. Step 2: Fix by removing Readonly

   Removing Readonly allows property changes. Options A, B, and D do not fix the core problem of property immutability.

3. Final Answer:

   Remove Readonly to allow property changes -> Option C

4. Quick Check:

   Readonly blocks property assignment; remove it to fix [OK]

Hint: Remove Readonly to allow property updates [OK]

Common Mistakes:

• Trying to assign to readonly property instead of removing Readonly
• Using 'as const' which makes object deeply readonly
• Changing variable declaration without fixing immutability

5. Given this nested object type, how can you make all properties readonly including nested ones?

interface Settings {
  theme: string;
  layout: {
    header: boolean;
    footer: boolean;
  };
}

hard

A. Use a custom DeepReadonly type that recursively applies Readonly

B. Use Partial<Settings> to make properties readonly

C. Use readonly Settings keyword

D. Use Readonly<Settings> only

Solution

1. Step 1: Understand shallow vs deep readonly

   The built-in Readonly utility only makes the top-level properties readonly. Nested objects remain mutable.

2. Step 2: Use a recursive DeepReadonly type

   To make nested properties readonly, you need a custom type that applies Readonly recursively to all nested objects.

3. Final Answer:

   Use a custom DeepReadonly type that recursively applies Readonly -> Option A

4. Quick Check:

   Readonly is shallow; use DeepReadonly for nested [OK]

Hint: Readonly is shallow; use recursive type for nested readonly [OK]

Common Mistakes:

• Assuming Readonly makes nested properties readonly
• Using 'readonly' keyword incorrectly on types
• Confusing Partial with Readonly