Readonly class properties in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

Typescript

class Person {
  readonly name: string;
  constructor(name: string) {
    this.[1] = name;
  }
}

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

Aname

Bage

Creadonly

Dthis

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to assign to a property not declared in the class.

Using readonly keyword in the constructor assignment.

The readonly property name is assigned inside the constructor using this.name = name;.

2fill in blank

medium

Complete the code to prevent modification of a readonly property after initialization.

Typescript

class Car {
  readonly model: string;
  constructor(model: string) {
    this.model = model;
  }

  changeModel(newModel: string) {
    // This line should cause an error
    this.model [1] newModel;
  }
}

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

A=

B+=

C===

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using comparison operators instead of assignment.

Trying to modify readonly property without error.

Assigning a new value to a readonly property like this.model = newModel; causes a TypeScript error.

3fill in blank

hard

Fix the error in the class by correctly declaring a readonly property with an initial value.

Typescript

class Book {
  [1] title: string = "Unknown";
}

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

Aconst

Breadonly

Cprivate

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using const inside a class property declaration.

Confusing private with readonly.

The readonly keyword declares a property that cannot be changed after initialization.

4fill in blank

hard

Fill both blanks to create a readonly property with a default value and a method to read it.

Typescript

class User {
  [1] id: number = 0;

  getId() {
    return this.[2];
  }
}

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

Areadonly

Bprivate

Cid

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using private instead of readonly.

Returning a property name that does not exist.

The property id is declared readonly and returned by the getId method.

5fill in blank

hard

Fill all three blanks to declare a readonly property, initialize it via constructor shorthand, and access it in a method.

Typescript

class Product {
  constructor([1] readonly [2]: string) {}

  display() {
    console.log(this.[3]);
  }
}

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

Apublic

BproductName

Dname

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for the property in constructor and method.

Omitting the readonly keyword in constructor shorthand.

The constructor uses shorthand to declare a readonly property productName. The display method accesses it with this.productName.

Practice

1. What does the readonly keyword do when used on a class property in TypeScript?

easy

A. It allows the property to be changed anytime.

B. It makes the property private and inaccessible outside the class.

C. It prevents the property from being changed after initialization.

D. It deletes the property after the constructor runs.

Solution

1. Step 1: Understand the purpose of readonly

   The readonly keyword marks a property so it cannot be changed after it is set initially.

2. Step 2: Check when assignment is allowed

   Assignment to a readonly property is only allowed during declaration or inside the constructor.

3. Final Answer:

   It prevents the property from being changed after initialization. -> Option C

4. Quick Check:

   Readonly means fixed value after set [OK]

Hint: Readonly means value can't change after constructor [OK]

Common Mistakes:

• Thinking readonly makes property private
• Assuming readonly allows changes anytime
• Confusing readonly with deleting properties

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

easy

A. id: readonly number;

B. readonly id: number;

C. let readonly id: number;

D. const id: number;

Solution

1. Step 1: Recall TypeScript readonly syntax

   The correct syntax places readonly before the property name and type.

2. Step 2: Check each option

   readonly id: number; matches the correct syntax: readonly id: number;. Others misuse keywords or order.

3. Final Answer:

   readonly id: number; -> Option B

4. Quick Check:

   Readonly keyword before property name [OK]

Hint: Put readonly before property name, not after type [OK]

Common Mistakes:

• Using const inside class properties
• Placing readonly after the type
• Combining let with readonly incorrectly

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

class User {
  readonly name: string;
  constructor(name: string) {
    this.name = name;
  }
  changeName(newName: string) {
    // @ts-ignore
    this.name = newName;
  }
}
const user = new User('Alice');
user.changeName('Bob');
console.log(user.name);

medium

A. Alice

B. undefined

C. Compilation error

D. Bob

Solution

1. Step 1: Understand readonly assignment rules

   The name property is readonly and assigned in the constructor, so initial value is 'Alice'. TypeScript enforces this at compile-time.

2. Step 2: Analyze the changeName method

   The @ts-ignore suppresses the TypeScript compile error for assigning to readonly. The code compiles to plain JavaScript, where objects are mutable, so this.name = newName; sets it to 'Bob' at runtime.

3. Final Answer:

   Bob -> Option D

4. Quick Check:

   Readonly is compile-time only; runtime allows changes [OK]

Hint: Readonly bypassed at runtime via @ts-ignore [OK]

Common Mistakes:

• Thinking readonly prevents runtime changes
• Expecting compilation error despite @ts-ignore
• Believing output remains 'Alice'

4. Identify the error in this TypeScript class:

class Product {
  readonly price: number;
  constructor() {
    // No assignment to price here
  }
}
const p = new Product();
p.price = 100;

medium

A. Error: readonly property 'price' is not initialized.

B. Error: Cannot assign to 'price' outside constructor.

C. No error, code runs fine.

D. Error: price must be declared as private.

Solution

1. Step 1: Check readonly property initialization

   The readonly property price must be assigned during declaration or in the constructor.

2. Step 2: Analyze constructor and assignment

   The constructor does not assign price, so TypeScript reports an error about missing initialization.

3. Final Answer:

   Error: readonly property 'price' is not initialized. -> Option A

4. Quick Check:

   Readonly props must be initialized at declaration or constructor [OK]

Hint: Always initialize readonly properties in constructor or declaration [OK]

Common Mistakes:

• Trying to assign readonly property after construction
• Ignoring missing initialization error
• Confusing readonly with private

5. Given this class, how can you safely allow updating the version property while keeping id readonly?

class Document {
  readonly id: string;
  version: number;
  constructor(id: string, version: number) {
    this.id = id;
    this.version = version;
  }
}

hard

A. Keep id readonly and update version normally via a method.

B. Make both id and version readonly and update via constructor only.

C. Remove readonly from id to allow all updates.

D. Make version readonly and update id instead.

Solution

1. Step 1: Understand readonly usage

   The id should remain readonly to keep it fixed after creation.

2. Step 2: Allow version updates

   The version property is not readonly, so it can be updated safely anytime, for example via a method.

3. Final Answer:

   Keep id readonly and update version normally via a method. -> Option A

4. Quick Check:

   Readonly for fixed, normal for changeable [OK]

Hint: Readonly for fixed props, normal for changeable ones [OK]

Common Mistakes:

• Making all properties readonly and losing flexibility
• Removing readonly from fixed properties
• Confusing which property should be updated