Optional properties in interfaces in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an optional property in the interface.

Typescript

interface User {
  name: string;
  age[1]: number;
}

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

A!

B:

C;

D?

Attempts:

3 left

💡 Hint

Common Mistakes

Using colon ':' instead of question mark '?' for optional properties.

Forgetting to add any symbol, which makes the property required.

In TypeScript, optional properties in interfaces are marked with a question mark ? after the property name.

2fill in blank

medium

Complete the code to define an interface with an optional property.

Typescript

interface Product {
  id: number;
  description[1]: string;
}

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

A:

B;

C?

D!

Attempts:

3 left

💡 Hint

Common Mistakes

Using colon ':' which is for type annotation, not optional marking.

Using exclamation mark '!' which is for definite assignment assertions.

The question mark ? after description marks it as optional in the interface.

3fill in blank

hard

Fix the error in the interface by correctly marking the optional property.

Typescript

interface Settings {
  theme: string;
  fontSize[1]: number;
}

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

A?

B:

C;

D!

Attempts:

3 left

💡 Hint

Common Mistakes

Using colon ':' which is for type annotation, not optional marking.

Using exclamation mark '!' which is for definite assignment assertions.

The property fontSize should be marked optional with a question mark ? to indicate it may be missing.

4fill in blank

hard

Complete the code to create an interface with one required and one optional property.

Typescript

interface Car {
  model:: string;
  year[1]: number;
}

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

A:

B?

C;

D!

Attempts:

3 left

💡 Hint

Common Mistakes

Marking both properties as optional or both as required.

Using semicolon or exclamation mark instead of colon or question mark.

The required property model uses a colon : for type annotation. The optional property year uses a question mark ? to mark it optional.

5fill in blank

hard

Complete the code to define an interface with one optional and two required properties.

Typescript

interface Book {
  title:: string;
  author:: string;
  pages[1]: number;
}

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

A?

B:

C;

D!

Attempts:

3 left

💡 Hint

Common Mistakes

Marking required properties as optional or vice versa.

Using semicolon or exclamation mark instead of colon or question mark.

The properties title and author are required, so they use colon :. The property pages is optional, so it uses a question mark ?.

Practice

1. What does the ? symbol mean when used in a TypeScript interface property like age?: number?

easy

A. The property is readonly and cannot be changed after initialization.

B. The property is required and must always be provided.

C. The property is private and cannot be accessed outside the interface.

D. The property is optional and may be omitted when creating an object.

Solution

1. Step 1: Understand the ? in interface properties

   The ? after a property name in an interface means that property is optional.

2. Step 2: Effect on object creation

   This means when creating an object of that interface type, you can omit that property without error.

3. Final Answer:

   The property is optional and may be omitted when creating an object. -> Option D

4. Quick Check:

   Optional property = may be omitted [OK]

Hint: Look for ? to spot optional properties quickly [OK]

Common Mistakes:

• Thinking ? means property is required
• Confusing optional with readonly or private
• Assuming optional properties must have default values

2. Which of the following is the correct syntax to declare an optional property email of type string in a TypeScript interface?

easy

A. email: string;

B. email: string?;

C. email?: string;

D. optional email: string;

Solution

1. Step 1: Recall optional property syntax

   In TypeScript, optional properties use a ? immediately after the property name.

2. Step 2: Match syntax to options

   Only email?: string; uses email?: string; which is correct syntax for optional property.

3. Final Answer:

   email?: string; -> Option C

4. Quick Check:

   Optional property syntax = name? : type [OK]

Hint: Remember: ? goes right after property name [OK]

Common Mistakes:

• Placing ? after the type instead of property name
• Using keywords like 'optional' which are invalid
• Omitting the semicolon but that is less critical

3. Given the interface and object below, what will be the output of console.log(user.age)?

interface User {
  name: string;
  age?: number;
}
const user: User = { name: "Alice" };
console.log(user.age);

medium

A. Error: Property 'age' is missing

B. undefined

C. null

D. 0

Solution

1. Step 1: Understand optional property behavior

   The property age is optional and not provided in the object.

2. Step 2: Check value of missing optional property

   Accessing a missing optional property returns undefined in JavaScript/TypeScript.

3. Final Answer:

   undefined -> Option B

4. Quick Check:

   Missing optional property = undefined [OK]

Hint: Missing optional property returns undefined, not error [OK]

Common Mistakes:

• Expecting runtime error for missing optional property
• Confusing undefined with null or zero
• Assuming default values are assigned automatically

4. Identify the error in the following code snippet:

interface Product {
  id: number;
  description?: string;
}

const item: Product = {
  id: 101,
  description: undefined
};

medium

A. Assigning undefined to an optional property is allowed.

B. Optional properties cannot be assigned undefined explicitly.

C. Missing semicolon after interface declaration.

D. Property 'id' should be optional too.

Solution

1. Step 1: Check assignment to optional property

   Optional properties can be omitted or explicitly assigned undefined.

2. Step 2: Validate code correctness

   Assigning undefined to description is valid and causes no error.

3. Final Answer:

   Assigning undefined to an optional property is allowed. -> Option A

4. Quick Check:

   Optional property can be undefined explicitly [OK]

Hint: Optional properties accept undefined explicitly without error [OK]

Common Mistakes:

• Thinking undefined assignment causes error
• Confusing optional with required properties
• Ignoring that missing semicolon is not an error here

5. You have this interface:

interface Config {
  url: string;
  timeout?: number;
  retries?: number;
}

Which of the following objects correctly implements Config with only url provided and no errors?

hard

A. { url: "http://example.com", timeout: undefined, retries: undefined }

B. { url: "http://example.com", timeout: null }

C. { url: "http://example.com", timeout: 0, retries: 3 }

D. { timeout: 1000, retries: 3 }

Solution

1. Step 1: Check required properties

   The url property is required and must be present.

2. Step 2: Validate optional properties usage

   Optional properties can be omitted or set to undefined without error.

3. Step 3: Analyze each option

   { url: "http://example.com", timeout: undefined, retries: undefined } has url and optional properties set to undefined, which is valid.
   { url: "http://example.com", timeout: null } uses null which is not assignable to number.
   { url: "http://example.com", timeout: 0, retries: 3 } includes optional properties but that is allowed, but question asks for only url provided.
   { timeout: 1000, retries: 3 } misses required url property.

4. Final Answer:

   { url: "http://example.com", timeout: undefined, retries: undefined } -> Option A

5. Quick Check:

   Required present, optional can be undefined or omitted [OK]

Hint: Required must exist; optional can be undefined or omitted [OK]

Common Mistakes:

• Using null for number type optional properties
• Omitting required properties
• Assuming optional must be omitted, not undefined