Readonly utility type in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this TypeScript code using Readonly?

Consider the following TypeScript code using the Readonly utility type. What will be logged to the console?

Typescript

interface User {
  name: string;
  age: number;
}

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

// user.age = 31; // Uncommenting this line causes an error

console.log(user.age);

A undefined

B 31

C 30

D Compilation error

💡 Hint

Readonly makes all properties immutable, so you cannot change them after initialization.

The Readonly utility type makes all properties of the object immutable. The code logs the original age value 30. Trying to assign a new value to user.age would cause a compilation error, but since that line is commented out, the code runs and logs 30.

🧠 Conceptual

intermediate

What does the Readonly utility type do in TypeScript?

Choose the best description of what the Readonly utility type does.

A It makes all properties of a type optional.

B It converts all properties of a type to strings.

C It removes all properties from a type.

D It makes all properties of a type immutable (read-only).

💡 Hint

Think about what 'readonly' means in everyday life.

Readonly utility type marks all properties of a type as immutable, meaning you cannot change their values after initialization.

🔧 Debug

advanced

What error does this code produce?

Examine this TypeScript code snippet. What error will the compiler show?

Typescript

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

const person: Readonly<Person> = { name: "Bob", age: 25 };
person.age = 26;

A SyntaxError: Unexpected token '='.

B Cannot assign to 'age' because it is a read-only property.

C No error, code runs fine.

D ReferenceError: person is not defined.

💡 Hint

Readonly properties cannot be assigned new values.

The Readonly utility type makes all properties read-only. Trying to assign a new value to person.age causes a TypeScript compile-time error indicating the property is read-only.

🚀 Application

advanced

How to make a nested object fully readonly?

Given this nested object type, which approach correctly makes all nested properties readonly in TypeScript?

Typescript

type Profile = {
  user: {
    name: string;
    address: {
      city: string;
      zip: number;
    };
  };
};

A

type ReadonlyProfile = {
  readonly user: {
    readonly name: string;
    readonly address: {
      readonly city: string;
      readonly zip: number;
    };
  };
};

B type ReadonlyProfile = Readonly<Profile>;

C type ReadonlyProfile = Readonly<Profile> & Readonly<Profile['user']> & Readonly<Profile['user']['address']>;

D type ReadonlyProfile = Readonly<Profile> & { user: Readonly<Profile['user']> };

💡 Hint

Readonly only applies shallowly by default.

The Readonly utility type only makes the first level properties readonly. To make nested properties readonly, you must explicitly mark each nested property as readonly or create a deep readonly type. Option A explicitly marks all nested properties readonly.

❓ Predict Output

expert

What is the output of this code using Readonly and mutation?

Analyze this TypeScript code. What will be the output when running it in JavaScript after compilation?

Typescript

type Data = {
  value: number;
  nested: {
    count: number;
  };
};

const data: Readonly<Data> = { value: 10, nested: { count: 5 } };

// data.value = 20; // Error: cannot assign to readonly

data.nested.count = 6;

console.log(data.nested.count);

A 6

B 5

C Compilation error

D Runtime error

💡 Hint

Readonly is shallow; nested objects are not deeply frozen.

Readonly only applies to the first level properties. The nested object itself is not readonly, so its properties can be changed. The code changes nested.count from 5 to 6 and logs 6.

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