Optional properties in Typescript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing optional property

What is the output of this TypeScript code?

Typescript

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

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

A null

B 0

C undefined

D TypeError

💡 Hint

Optional properties may not exist on the object.

The property 'age' is optional and not set, so accessing it returns undefined.

❓ Predict Output

intermediate

Output when optional property is checked

What will this code output?

Typescript

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

const product: Product = { id: 101 };
if (product.description) {
  console.log(product.description.length);
} else {
  console.log("No description");
}

A 0

B No description

C undefined

D TypeError

💡 Hint

Check if optional property exists before accessing.

Since 'description' is undefined, the else branch runs printing 'No description'.

🔧 Debug

advanced

Why does this code cause an error?

This TypeScript code causes a compilation error. What is the reason?

Typescript

interface Config {
  timeout?: number;
}

function setTimeoutValue(config: Config) {
  const time = config.timeout + 1000;
  console.log(time);
}

setTimeoutValue({});

A Syntax error in interface declaration

B 'timeout' is missing and cannot be accessed

C Function setTimeoutValue requires timeout property to be present

D Property 'timeout' might be undefined, so adding number causes error

💡 Hint

Optional properties can be undefined and need checks before use.

Since 'timeout' can be undefined, adding 1000 to it without checking causes a type error.

📝 Syntax

advanced

Which option correctly defines an optional property?

Which of these interface property declarations correctly defines an optional property named 'email' of type string?

A email?: string;

B email: string?;

C email: string | undefined;

D optional email: string;

💡 Hint

Optional properties use a special syntax with a question mark after the name.

The correct syntax for optional properties is placing '?' after the property name before the colon.

🚀 Application

expert

How many keys are in the resulting object?

Given this code, how many keys does the object 'settings' have after execution?

Typescript

interface Settings {
  theme?: string;
  fontSize?: number;
  language?: string;
}

const settings: Settings = { theme: "dark" };

if (settings.fontSize === undefined) {
  settings.fontSize = 14;
}

if (!settings.language) {
  settings.language = "en";
}

console.log(Object.keys(settings).length);

A 3

B 1

C 0

D 2

💡 Hint

Optional properties can be added later by assignment.

Initially only 'theme' is set. Then 'fontSize' and 'language' are assigned, so total keys are 3.

Practice

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

easy

A. The property is required and must always be present.

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

C. The property is optional and may or may not be present in objects.

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

Solution

1. Step 1: Understand the meaning of ? in interfaces

   The ? after a property name marks it as optional, meaning it can be missing in objects that implement the interface.

2. Step 2: Compare with other property types

   Required properties must always be present, private and readonly are unrelated concepts.

3. Final Answer:

   The property is optional and may or may not be present in objects. -> Option C

4. Quick Check:

   Optional property = property with ? [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 a default value

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. optional email: string

C. email: string?

D. email: optional string

Solution

1. Step 1: Recall TypeScript optional property syntax

   Optional properties use a ? immediately after the property name, before the colon and type.

2. Step 2: Check each option

   email?: string matches the correct syntax: email?: string. Others are invalid syntax.

3. Final Answer:

   email?: string -> Option A

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
• Missing colon between property name and type

3. Given this interface and object:

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

const user: User = { id: 1 };
console.log(user.name);

What will be the output when running this code?

medium

A. undefined

B. null

C. An error because name is missing

D. Empty string

Solution

1. Step 1: Understand optional property behavior

   The property name is optional and not provided in the object, so it is undefined.

2. Step 2: Check console.log output

   Logging user.name prints undefined because the property is missing, not null or empty string.

3. Final Answer:

   undefined -> Option A

4. Quick Check:

   Missing optional property = undefined [OK]

Hint: Missing optional properties are undefined, not errors [OK]

Common Mistakes:

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

4. Identify the error in this TypeScript interface declaration:

interface Product {
  id: number;
  description?: string = "No description";
}

medium

A. The interface keyword is misspelled.

B. The property id must be optional too.

C. The equals sign = should be a colon :.

D. Optional properties cannot have default values in interfaces.

Solution

1. Step 1: Check interface property syntax

   Interfaces only declare types; they cannot assign default values to properties.

2. Step 2: Analyze the error

   The syntax description?: string = "No description" is invalid because default values are not allowed in interfaces.

3. Final Answer:

   Optional properties cannot have default values in interfaces. -> Option D

4. Quick Check:

   Interfaces declare types, no default values allowed [OK]

Hint: Interfaces declare types only, no default values [OK]

Common Mistakes:

• Trying to assign default values inside interfaces
• Confusing interfaces with classes or object literals
• Ignoring TypeScript syntax rules for interfaces

5. You want to create a function that accepts an object with optional middleName property and returns the full name. Which of these implementations correctly handles the optional property without errors?

interface Person {
  firstName: string;
  middleName?: string;
  lastName: string;
}

function getFullName(person: Person): string {
  // Which code below is correct?
}

hard

A. return `${person.firstName} ${person.middleName} ${person.lastName}`;

B. return `${person.firstName} ${person.middleName ?? ''} ${person.lastName}`.trim();

C. return `${person.firstName} ${person.middleName!} ${person.lastName}`;

D. return person.firstName + person.middleName + person.lastName;

Solution

1. Step 1: Understand optional property usage

   The middleName may be undefined, so accessing it directly can cause unwanted 'undefined' in output.

2. Step 2: Evaluate each option

   return `${person.firstName} ${person.middleName} ${person.lastName}`; inserts undefined if missing. return `${person.firstName} ${person.middleName ?? ''} ${person.lastName}`.trim(); uses nullish coalescing ?? to replace undefined with empty string and trims extra spaces. return `${person.firstName} ${person.middleName!} ${person.lastName}`; uses non-null assertion which can cause runtime errors if missing. return person.firstName + person.middleName + person.lastName; concatenates without spaces and can produce 'undefined' string.

3. Final Answer:

   return `${person.firstName} ${person.middleName ?? ''} ${person.lastName}`.trim(); -> Option B

4. Quick Check:

   Use ?? '' to safely handle optional strings [OK]

Hint: Use nullish coalescing (?? '') to handle optional strings safely [OK]

Common Mistakes:

• Directly using optional property without checks
• Using non-null assertion (!) unsafely
• Concatenating without spaces or trimming