Optional properties in interfaces in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing optional properties

What is the output of this TypeScript code when compiled and run with Node.js?

Typescript

interface User {
  name: string;
  age?: number;
}

const user1: User = { name: "Alice" };
console.log(user1.age);

A TypeError at runtime

B 0

C undefined

D null

💡 Hint

Optional properties may be missing and return undefined if not set.

The property age is optional, so it may not exist on the object. Accessing it returns undefined rather than an error or default value.

❓ Predict Output

intermediate

Output when optional property is assigned

What will be printed by this TypeScript code?

Typescript

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

const p: Product = { id: 101, description: "A book" };
console.log(p.description);

A "A book"

B undefined

C null

D Compilation error

💡 Hint

Optional means the property can be present or absent. Here it is present.

The description property is optional but is provided in the object. So, logging it prints the string value.

❓ Predict Output

advanced

Output of function using optional property

What is the output of this TypeScript code?

Typescript

interface Config {
  timeout?: number;
}

function getTimeout(config: Config): number {
  return config.timeout ?? 5000;
}

console.log(getTimeout({}));
console.log(getTimeout({ timeout: 1000 }));

A 5000\n1000

B undefined\n1000

C 0\n1000

D Compilation error

💡 Hint

The nullish coalescing operator returns the right side if the left is null or undefined.

The timeout property is optional. If missing, the function returns 5000 as default. If present, it returns the given value.

❓ Predict Output

advanced

Result of assigning object missing optional property

What will be the value of obj after this code runs?

Typescript

interface Settings {
  darkMode?: boolean;
}

const obj: Settings = {};
obj.darkMode = obj.darkMode ?? true;
console.log(obj.darkMode);

A undefined

B true

C false

D TypeError

💡 Hint

The nullish coalescing operator assigns true if the property is undefined.

Since darkMode is missing (undefined), the expression assigns true to it.

❓ Predict Output

expert

Output of nested optional properties access

What is the output of this TypeScript code?

Typescript

interface Address {
  street?: string;
}
interface Person {
  name: string;
  address?: Address;
}

const person: Person = { name: "Bob" };
console.log(person.address?.street ?? "No street");

A Runtime error

B undefined

C null

D "No street"

💡 Hint

Optional chaining safely accesses nested optional properties.

The address property is missing, so person.address?.street is undefined. The nullish coalescing operator then returns "No street".

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