Readonly utility type in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Readonly utility type do in TypeScript?

It makes all properties of an object type read-only, so they cannot be changed after initialization.

beginner

How do you apply the Readonly utility type to an interface Person?

You write Readonly<Person> to create a new type where all properties of Person are read-only.

beginner

Can you modify properties of an object typed with Readonly after it is created?

No, properties are read-only and TypeScript will give an error if you try to assign new values.

intermediate

Example: What is the error in this code?<br><pre>type User = { name: string };
const user: Readonly&lt;User&gt; = { name: 'Alice' };
user.name = 'Bob';</pre>

Error: Cannot assign to 'name' because it is a read-only property.

beginner

Why use Readonly utility type in your code?

To prevent accidental changes to objects, making code safer and easier to understand by enforcing immutability.

What does the Readonly utility type do?

A Converts properties to optional

B Makes all properties of an object read-only

C Allows properties to be changed anytime

D Removes all properties from an object

Readonly makes all properties read-only, preventing changes.

How do you declare a read-only version of a type Car?

A Readonly<Car>

B Mutable<Car>

C Partial<Car>

D Required<Car>

Readonly creates a type with all properties read-only.

What happens if you try to assign a new value to a read-only property?

A The value changes successfully

B The property becomes optional

C The property is deleted

D TypeScript shows an error

TypeScript prevents assignment to read-only properties with an error.

Which of these is a benefit of using Readonly?

A Prevents accidental data changes

B Makes code run faster

C Allows dynamic property addition

D Removes type checking

Readonly helps avoid accidental changes, improving code safety.

Can Readonly be used on nested objects automatically?

A It removes nested objects

B Yes, it applies deeply by default

C No, it only applies to the first level properties

D It converts nested objects to arrays

Readonly only makes the top-level properties read-only, nested objects remain mutable unless handled separately.

Explain what the Readonly utility type does and why it is useful.

Think about how it stops you from changing object properties after creation.

Describe how you would use Readonly with a TypeScript interface and what happens if you try to modify a property.

Consider the syntax and the error you get when changing a read-only property.

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