Optional properties in Typescript - Mini Project: Build & Apply

Optional properties

📖 Scenario: You are creating a simple contact list app. Some contacts have a phone number, but some do not.

🎯 Goal: Learn how to define and use optional properties in TypeScript interfaces.

📋 What You'll Learn

Create an interface with optional properties

Create objects using that interface

Access optional properties safely

💡 Why This Matters

🌍 Real World

Many real-world apps store user or contact information where some details may be missing. Optional properties let you model this clearly.

💼 Career

Understanding optional properties is important for writing robust TypeScript code in web development, especially when working with APIs or user data.

1

Create an interface with optional properties

Create an interface called Contact with two properties: name of type string and an optional property phone of type string. Use the ? symbol to mark phone as optional.

Typescript

# Your code here

Hint

Use phone?: string to make the phone property optional.

2

Create contact objects using the interface

Create two variables called contact1 and contact2 of type Contact. Set contact1 to have name as "Alice" and phone as "123-4567". Set contact2 to have name as "Bob" without a phone property.

Typescript

interface Contact {
  name: string;
  phone?: string;
}

# Your code here

Hint

Remember to omit the phone property for contact2.

3

Check and use optional properties

Write a function called printContact that takes a Contact parameter called contact. Inside the function, use an if statement to check if contact.phone exists. If it does, print "Name: " plus the name and "Phone: " plus the phone. Otherwise, print "Name: " plus the name and "Phone: Not available".

Typescript

interface Contact {
  name: string;
  phone?: string;
}

const contact1: Contact = { name: "Alice", phone: "123-4567" };
const contact2: Contact = { name: "Bob" };

# Your code here

Hint

Use if (contact.phone) to check if the phone exists.

4

Print contact details

Call printContact with contact1 and then with contact2 to display their details.

Typescript

interface Contact {
  name: string;
  phone?: string;
}

const contact1: Contact = { name: "Alice", phone: "123-4567" };
const contact2: Contact = { name: "Bob" };

function printContact(contact: Contact): void {
  if (contact.phone) {
    console.log(`Name: ${contact.name}, Phone: ${contact.phone}`);
  } else {
    console.log(`Name: ${contact.name}, Phone: Not available`);
  }
}

# Your code here

Hint

Call printContact(contact1) and printContact(contact2).

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