Readonly properties in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a readonly property in a TypeScript interface.

Typescript

interface User {
  readonly [1]: string;
}

Drag options to blanks, or click blank then click option'

Apassword

Bage

Cemail

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using a mutable property name without readonly

Trying to assign a value to a readonly property later

The readonly keyword makes the property immutable after initialization. Here, name is the readonly property.

2fill in blank

medium

Complete the code to create an object that respects the readonly property.

Typescript

const user: { readonly id: number } = { id: [1] };

Drag options to blanks, or click blank then click option'

A42

B'42'

Cnull

Dtrue

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning a string instead of a number

Trying to assign a boolean or null

The id property is a number and readonly. Assigning 42 as a number is correct.

3fill in blank

hard

Fix the error by completing the code to prevent modifying a readonly property.

Typescript

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

const p: Point = { x: 10, y: 20 };
p.[1] = 30;

Drag options to blanks, or click blank then click option'

AtoString

Bx

Cz

Dy

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to assign to a readonly property

Using a property name that does not exist

Properties x and y are readonly. Trying to assign p.x = 30 causes an error. The blank is the property name being assigned.

4fill in blank

hard

Fill both blanks to create a readonly property and assign it correctly.

Typescript

interface Config {
  [1] version: string;
}

const config: Config = { version: [2] };

Drag options to blanks, or click blank then click option'

Areadonly

B'1.0.0'

C"1.0.0"

Dconst

Attempts:

3 left

💡 Hint

Common Mistakes

Using const instead of readonly in interface

Assigning a string without quotes

The property version is declared readonly. The value assigned is a string literal with double quotes.

5fill in blank

hard

Fill all three blanks to create a readonly property, assign it, and try to modify it (which should cause an error).

Typescript

interface Settings {
  [1] theme: string;
}

const settings: Settings = { theme: [2] };
settings.[3] = "dark";

Drag options to blanks, or click blank then click option'

Areadonly

B"light"

Ctheme

Dconst

Attempts:

3 left

💡 Hint

Common Mistakes

Not using readonly keyword

Assigning a value without quotes

Trying to modify a readonly property

The property theme is readonly. It is assigned the string "light". Trying to assign settings.theme = "dark" causes an error because theme is readonly.

Practice

1. What does the readonly keyword do in TypeScript?

easy

A. Prevents a property from being changed after it is set

B. Allows a property to be changed anytime

C. Makes a property private and inaccessible

D. Deletes a property from an object

Solution

1. Step 1: Understand the purpose of readonly

   The readonly keyword is used to mark properties that cannot be changed after initialization.

2. Step 2: Compare options with the behavior

   Only Prevents a property from being changed after it is set correctly describes that the property cannot be changed after it is set.

3. Final Answer:

   Prevents a property from being changed after it is set -> Option A

4. Quick Check:

   readonly means no changes allowed after set [OK]

Hint: Readonly means no changes allowed after initial set [OK]

Common Mistakes:

• Thinking readonly allows changes anytime
• Confusing readonly with private
• Assuming readonly deletes properties

2. Which of the following is the correct way to declare a readonly property id in a TypeScript class?

easy

A. const id: number;

B. readonly id: number;

C. private readonly id: number;

D. id readonly: number;

Solution

1. Step 1: Recall TypeScript syntax for readonly properties

   The correct syntax is to place readonly before the property name.

2. Step 2: Check each option

   readonly id: number; uses readonly id: number; which is correct. const id: number; uses const which is invalid inside class properties. private readonly id: number; adds private which is valid but not asked for. id readonly: number; has wrong order.

3. Final Answer:

   readonly id: number; -> Option B

4. Quick Check:

   Readonly keyword goes before property name [OK]

Hint: Put readonly before property name in class [OK]

Common Mistakes:

• Using const instead of readonly in class
• Placing readonly after property name
• Confusing access modifiers with readonly

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

class User {
  readonly name: string;
  constructor(name: string) {
    this.name = name;
  }
}
const user = new User('Alice');
user.name = 'Bob';
console.log(user.name);

medium

A. Alice

B. Bob

C. undefined

D. Compile-time error

Solution

1. Step 1: Understand readonly property behavior

   The property name is marked readonly, so it cannot be assigned a new value after construction.

2. Step 2: Analyze the assignment user.name = 'Bob'

   This line tries to change a readonly property, which causes a compile-time error in TypeScript.

3. Final Answer:

   Compile-time error -> Option D

4. Quick Check:

   Assigning to readonly property causes error [OK]

Hint: Changing readonly property causes compile error [OK]

Common Mistakes:

• Assuming readonly properties can be changed anytime
• Expecting runtime error instead of compile error
• Thinking output will be the new value

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

interface Point {
  readonly x: number;
  y: number;
}
const p: Point = { x: 10, y: 20 };
p.x = 15;
console.log(p.x);

medium

A. Property 'y' should be readonly too

B. Missing type annotation for 'p'

C. Cannot assign to 'x' because it is a readonly property

D. No error, code runs fine

Solution

1. Step 1: Check readonly property assignment

   The property x is readonly in the interface, so assigning p.x = 15 is not allowed.

2. Step 2: Identify the error message

   TypeScript will report an error: "Cannot assign to 'x' because it is a readonly property".

3. Final Answer:

   Cannot assign to 'x' because it is a readonly property -> Option C

4. Quick Check:

   Readonly properties cannot be assigned after initialization [OK]

Hint: Readonly properties cannot be reassigned after creation [OK]

Common Mistakes:

• Ignoring readonly and trying to assign anyway
• Thinking missing type annotation causes error
• Assuming no error occurs

5. You want to create a TypeScript class Config where the version property is set once during construction and never changes, but settings can be updated anytime. Which is the best way to declare these properties?

hard

A. readonly version: string; settings: object;

B. version: string; readonly settings: object;

C. readonly version: string; readonly settings: object;

D. version: string; settings: object;

Solution

1. Step 1: Understand property requirements

   The version should be readonly (set once, no changes). The settings should be mutable (can change anytime).

2. Step 2: Match options with requirements

   readonly version: string; settings: object; correctly marks version as readonly and leaves settings mutable. version: string; readonly settings: object; reverses roles. readonly version: string; readonly settings: object; makes both readonly. version: string; settings: object; makes both mutable.

3. Final Answer:

   readonly version: string; settings: object; -> Option A

4. Quick Check:

   Readonly for fixed, mutable for changeable properties [OK]

Hint: Readonly for fixed, normal for changeable properties [OK]

Common Mistakes:

• Making mutable properties readonly by mistake
• Not using readonly for fixed properties
• Confusing which property should be readonly