Optional parameters in Typescript - Mini Project: Build & Apply

Using Optional Parameters in TypeScript Functions

📖 Scenario: You are building a simple greeting system for a website. Sometimes users provide their middle name, but sometimes they do not. You want to create a function that can greet users with or without their middle name.

🎯 Goal: Create a TypeScript function with an optional parameter to greet users by their full name or just first and last name.

📋 What You'll Learn

Create a function called greetUser with three parameters: firstName, lastName, and an optional middleName.

Set the middleName parameter as optional using TypeScript syntax.

Use the function to return a greeting string that includes the middle name if it is provided, or just first and last name if not.

Print the greeting for a user with a middle name and a user without a middle name.

💡 Why This Matters

🌍 Real World

Optional parameters are useful when you want to make your functions flexible. For example, greeting users with or without a middle name is common in real apps.

💼 Career

Understanding optional parameters is important for writing clean, reusable TypeScript code in many software development jobs.

1

Create the greetUser function with required parameters

Write a function called greetUser that takes two parameters: firstName and lastName, both of type string. The function should return a string greeting in the format: "Hello, firstName lastName!".

Typescript

function greetUser(firstName: string, lastName: string): string {
  // Your code here
}

Hint

Use backticks ` to create a template string for the greeting.

2

Add an optional middleName parameter

Modify the greetUser function to add a third parameter called middleName of type string. Make this parameter optional using TypeScript syntax.

Typescript

function greetUser(firstName: string, lastName: string, middleName?: string): string {
  // Your code here
}

Hint

Add a question mark ? after middleName to make it optional.

3

Use the optional middleName in the greeting

Update the greetUser function to include the middleName in the greeting if it is provided. If middleName is not given, return the greeting with just firstName and lastName. Use an if statement to check if middleName exists.

Typescript

function greetUser(firstName: string, lastName: string, middleName?: string): string {
  // Your code here
}

Hint

Use an if statement to check if middleName is truthy before including it.

4

Print greetings with and without the optional parameter

Call the greetUser function twice: once with firstName = "John", middleName = "Paul", and lastName = "Smith"; and once with firstName = "Jane" and lastName = "Doe" only. Print both results using console.log.

Typescript

console.log(greetUser("John", "Smith", "Paul"));
// Your code here

Hint

Remember to call console.log for each greeting.

Practice

1.

What does it mean when a function parameter in TypeScript is marked as optional with a ??

easy

A. The parameter is automatically set to zero.

B. You can call the function without providing that parameter.

C. The parameter is ignored inside the function.

D. The parameter must always be provided when calling the function.

Solution

1. Step 1: Understand optional parameter meaning

   In TypeScript, a parameter marked with ? means it is optional and can be skipped when calling the function.

2. Step 2: Compare with required parameters

   Required parameters must be given, but optional ones can be left out without error.

3. Final Answer:

   You can call the function without providing that parameter. -> Option B

4. Quick Check:

   Optional parameter means you can skip it [OK]

Hint: Optional parameters can be left out when calling functions [OK]

Common Mistakes:

• Thinking optional means parameter is ignored
• Assuming optional parameters must be last but not knowing why
• Confusing optional with default parameters

2.

Which of the following is the correct way to declare a function with an optional parameter age of type number in TypeScript?

easy

A. function greet(name: string, age: number?) {}

B. function greet(name: string, ?age: number) {}

C. function greet(name: string, age?: number) {}

D. function greet(name: string, age: optional number) {}

Solution

1. Step 1: Recall optional parameter syntax

   In TypeScript, optional parameters are declared by adding ? after the parameter name, before the colon and type.

2. Step 2: Check each option

   function greet(name: string, age?: number) {} uses age?: number, which is correct. Options B, C, and D use invalid syntax.

3. Final Answer:

   function greet(name: string, age?: number) {} -> Option C

4. Quick Check:

   Optional parameter syntax = paramName?: type [OK]

Hint: Use ? right after parameter name for optional [OK]

Common Mistakes:

• Placing ? after the type instead of the name
• Using invalid keywords like 'optional'
• Putting ? before parameter name

3.

What will be the output of this TypeScript code?

function multiply(a: number, b?: number) {
  return b ? a * b : a * 2;
}
console.log(multiply(5));
console.log(multiply(5, 3));

medium

A. 10 and 15

B. 5 and 15

C. 10 and 8

D. Error because b is optional

Solution

1. Step 1: Analyze function behavior with optional parameter

   If b is provided, return a * b. Otherwise, return a * 2.

2. Step 2: Evaluate calls

   First call: multiply(5) means b is undefined, so returns 5 * 2 = 10. Second call: multiply(5, 3) returns 5 * 3 = 15.

3. Final Answer:

   10 and 15 -> Option A

4. Quick Check:

   Optional param missing uses default logic [OK]

Hint: Optional param missing triggers alternate logic in function [OK]

Common Mistakes:

• Assuming missing optional param causes error
• Confusing default value with optional parameter
• Misreading ternary condition

4.

Identify the error in this TypeScript function declaration:

function logMessage(message?: string, level: string) {
  console.log(`[${level}] ${message}`);
}

medium

A. Optional parameter must be last in the parameter list.

B. Optional parameters cannot be strings.

C. Function must return a value if it has optional parameters.

D. No error, this is valid syntax.

Solution

1. Step 1: Recall rule about optional parameters order

   In TypeScript, optional parameters must come after all required parameters.

2. Step 2: Check parameter order

   Here, message? is optional but placed before required level. This causes a syntax error.

3. Final Answer:

   Optional parameter must be last in the parameter list. -> Option A

4. Quick Check:

   Optional params must be last [OK]

Hint: Put all optional parameters after required ones [OK]

Common Mistakes:

• Placing optional parameters before required ones
• Thinking optional means default value
• Ignoring TypeScript syntax rules

5.

You want to write a function createUser that takes a required username and an optional age. If age is not provided, it should default to 18. Which of these is the best way to declare the function?

hard

A. function createUser(username: string, age?: number = 18) {}

B. function createUser(username: string, age?: number) { age = 18; }

C. function createUser(username: string, age?: number) { if (!age) age = 18; }

D. function createUser(username: string, age: number = 18) {}

Solution

1. Step 1: Understand difference between optional and default parameters

   Optional parameters can be omitted but have no default value unless assigned. Default parameters have a default value and are optional by nature.

2. Step 2: Check syntax and behavior

   function createUser(username: string, age: number = 18) {} correctly uses a default parameter age: number = 18, which means if age is omitted, it defaults to 18. function createUser(username: string, age?: number = 18) {} (age?: number = 18) is valid syntax but the ? is redundant since default parameters are already optional. Options B and D do not set default properly and may cause issues.

3. Final Answer:

   function createUser(username: string, age: number = 18) {} -> Option D

4. Quick Check:

   Use default parameter for optional with default value [OK]

Hint: Use default parameter syntax for optional with default [OK]

Common Mistakes:

• Using redundant ? with default parameter syntax
• Setting default inside function body incorrectly
• Confusing optional and default parameters