Readonly properties in interfaces in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

Typescript

interface User {
  [1] id: number;
}

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

Aprivate

Bconst

Cstatic

Dreadonly

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'const' inside interfaces is invalid.

Using 'static' or 'private' does not make properties readonly.

The readonly keyword makes the property immutable after initialization.

2fill in blank

medium

Complete the code to create an interface with a readonly property 'name'.

Typescript

interface Person {
  [1] name: string;
}

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

Areadonly

Bprivate

Cpublic

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'private' instead of 'readonly'.

Forgetting to add any modifier.

The readonly keyword ensures the 'name' property cannot be changed after assignment.

3fill in blank

hard

Fix the error in the interface by making the property 'age' readonly.

Typescript

interface Employee {
  [1] age: number;
}

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

Areadonly

Bstatic

Cconst

Dprivate

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'const' inside interfaces.

Using 'private' or 'static' incorrectly.

Only readonly is valid to make interface properties immutable.

4fill in blank

hard

Fill both blanks to declare a readonly property 'title' of type string in the interface.

Typescript

interface Book {
  [1] [2]: string;
}

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

Areadonly

Bprivate

Ctitle

Dstatic

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping the order of keyword and property name.

Using 'private' or 'static' instead of 'readonly'.

The property name is 'title' and it must be marked readonly to prevent changes.

5fill in blank

hard

Fill all three blanks to declare a readonly property 'id' of type number in the interface.

Typescript

interface Product {
  [1] [2]: [3];
}

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

Areadonly

Bid

Cnumber

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' instead of 'number' for the type.

Omitting the readonly keyword.

The property 'id' is marked readonly and typed as number.

Practice

1.

What does the readonly keyword do when used in a TypeScript interface property?

easy

A. Prevents the property from being changed after initialization

B. Allows the property to be changed anytime

C. Makes the property optional

D. Makes the property private

Solution

1. Step 1: Understand the purpose of readonly in interfaces

   The readonly keyword marks a property so it cannot be reassigned after the object is created.

2. Step 2: Compare with other options

   Options A, B, and D describe different behaviors: A makes optional, B allows changes, D changes visibility, none of which match readonly.

3. Final Answer:

   Prevents the property from being changed after initialization -> Option A

4. Quick Check:

   readonly means no changes allowed after creation [OK]

Hint: Readonly means no changes allowed after creation [OK]

Common Mistakes:

• Thinking readonly makes property optional
• Confusing readonly with private or protected
• Assuming readonly allows reassignment

2.

Which of the following is the correct syntax to declare a readonly property id of type number in a TypeScript interface?

interface User {
  ?
}

easy

A. id: readonly number;

B. id readonly: number;

C. readonly: id number;

D. readonly id: number;

Solution

1. Step 1: Recall correct readonly syntax in interfaces

   The correct syntax places readonly before the property name, then a colon and the type.

2. Step 2: Check each option

   readonly id: number; matches the correct syntax. Options A, B, and C have incorrect order or misplaced keywords.

3. Final Answer:

   readonly id: number; -> Option D

4. Quick Check:

   Readonly keyword comes before property name [OK]

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

Common Mistakes:

• Placing readonly after property name
• Using readonly as a type
• Misplacing colons or semicolons

3.

Consider the following code:

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

const p: Point = { x: 10, y: 20 };
p.x = 30;
console.log(p.x);

What will happen when this code runs?

medium

A. It will print 30

B. It will print 10

C. Compile-time error due to assignment to readonly property

D. Runtime error when assigning to readonly property

Solution

1. Step 1: Understand readonly behavior at compile time

   TypeScript prevents assigning a new value to a readonly property during compilation, causing an error before running.

2. Step 2: Check runtime behavior

   Since TypeScript errors stop compilation, the code won't run, so no runtime error or output occurs.

3. Final Answer:

   Compile-time error due to assignment to readonly property -> Option C

4. Quick Check:

   Readonly blocks assignment at compile time [OK]

Hint: Readonly errors happen at compile time, not runtime [OK]

Common Mistakes:

• Thinking readonly allows reassignment at runtime
• Expecting runtime error instead of compile error
• Assuming original value changes silently

4.

Find the error in this code snippet:

interface Config {
  readonly url: string;
}

const config: Config = { url: "https://api.com" };
config.url = "https://newapi.com";

medium

A. Cannot assign to 'url' because it is a readonly property

B. Missing semicolon after interface declaration

C. Type mismatch: string assigned to readonly property

D. Interface properties cannot be initialized

Solution

1. Step 1: Identify the readonly property usage

   The property url is marked readonly, so it cannot be reassigned after initialization.

2. Step 2: Check the assignment line

   The line config.url = "https://newapi.com"; tries to change a readonly property, causing a compile-time error.

3. Final Answer:

   Cannot assign to 'url' because it is a readonly property -> Option A

4. Quick Check:

   Readonly properties cannot be reassigned [OK]

Hint: Readonly properties cause errors if reassigned [OK]

Common Mistakes:

• Ignoring readonly and trying to reassign
• Confusing readonly with optional properties
• Thinking semicolon or type is the error

5.

You want to create an interface Person with a readonly id and a mutable name. Which code correctly enforces this?

?

hard

A. interface Person { id: readonly number; name: string; }

B. interface Person { readonly id: number; name: string; }

C. interface Person { readonly id: number; readonly name: string; }

D. interface Person { id: number; name: readonly string; }

Solution

1. Step 1: Define readonly and mutable properties correctly

   The readonly keyword must be before the property name to make it immutable; properties without it remain mutable.

2. Step 2: Evaluate each option

   interface Person { readonly id: number; name: string; } correctly marks id as readonly and name as mutable. Options B, C, and D misuse readonly or make both properties readonly.

3. Final Answer:

   interface Person { readonly id: number; name: string; } -> Option B

4. Quick Check:

   Readonly before property name only for immutable fields [OK]

Hint: Put readonly before only the properties you want fixed [OK]

Common Mistakes:

• Placing readonly after property name or type
• Making all properties readonly by mistake
• Confusing readonly with type annotations