Readonly properties in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the readonly keyword do in TypeScript?

It makes a property unchangeable after it is initialized. You can set it once, but cannot modify it later.

beginner

How do you declare a readonly property in a TypeScript interface?

Use the readonly keyword before the property name, like readonly name: string;.

intermediate

Can you assign a value to a readonly property inside a class constructor?

Yes, you can assign a readonly property inside the constructor, but not modify it outside.

beginner

What happens if you try to modify a readonly property after initialization?

TypeScript will give a compile-time error saying you cannot assign to a readonly property.

intermediate

Can readonly properties be used with arrays or objects to make them immutable?

Readonly properties prevent reassigning the property itself, but do not make the contents of arrays or objects immutable.

What does the readonly keyword do in TypeScript?

A Prevents modification of a property after initialization

B Makes a property optional

C Allows a property to be changed anytime

D Deletes a property from an object

The readonly keyword ensures a property cannot be changed after it is first set.

Where can you assign a value to a readonly property in a class?

A Only inside the constructor

B Anywhere in the class

C Only outside the class

D Nowhere, it must be undefined

Readonly properties can be assigned or changed inside the constructor but not elsewhere.

What error occurs if you try to change a readonly property after initialization?

A Syntax error

B Runtime error

C Compile-time error

D No error, it changes normally

TypeScript gives a compile-time error to prevent changing readonly properties.

How do you declare a readonly property in an interface?

A Use <code>const</code> before the property name

B Use <code>readonly</code> before the property name

C Use <code>private</code> before the property name

D Use <code>static</code> before the property name

The readonly keyword is used before the property name in interfaces.

Does readonly make the contents of an array property immutable?

A Yes, but only for string arrays

B Yes, it makes the array contents immutable

C No, it deletes the array contents

D No, it only prevents reassigning the property itself

Readonly prevents changing the property reference, but array elements can still be changed.

Explain how readonly properties work in TypeScript and where you can assign their values.

Think about when and where you can set a readonly property.

Describe the difference between readonly properties and immutability of objects or arrays in TypeScript.

Consider what readonly protects and what it does not.

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