Optional parameters in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an optional parameter in the function.

Typescript

function greet(name[1] string) {
  console.log(`Hello, ${name}!`);
}

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

A? :

B:?

C?:

D?:=

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the question mark after the colon.

Using spaces between the question mark and colon.

Using an equal sign with the question mark.

In TypeScript, optional parameters are declared by adding a question mark immediately after the parameter name, before the colon.

2fill in blank

medium

Complete the function call to use the optional parameter correctly.

Typescript

function greet(name?: string) {
  if (name) {
    console.log(`Hello, ${name}!`);
  } else {
    console.log('Hello!');
  }
}

greet([1]);

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

A'Alice'

Bundefined

Cnull

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Passing null which is not the same as undefined in TypeScript optional parameters.

Passing 0 which is a number, not a string.

Passing undefined when the goal is to provide a name.

Passing a string like 'Alice' uses the optional parameter. Passing undefined also works but here we want to show usage with a name.

3fill in blank

hard

Fix the error in the function declaration to make the second parameter optional.

Typescript

function multiply(a: number, b[1] number): number {
  return a * b;
}

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

A?:

B?:=

C:?

D? :

Attempts:

3 left

💡 Hint

Common Mistakes

Putting the question mark after the colon.

Using spaces between the question mark and colon.

Using an equal sign with the question mark.

The correct syntax for optional parameters is to place a question mark immediately after the parameter name, before the colon.

4fill in blank

hard

Fill both blanks to create a function with an optional parameter and a default value.

Typescript

function greet(name[1] string[2] = 'Guest') {
  console.log(`Hello, ${name}!`);
}

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

A?:

B=

C ?=

D:

Attempts:

3 left

💡 Hint

Common Mistakes

Using ?= which is not valid syntax.

Putting = before the type.

Omitting the question mark for optional parameters.

Optional parameters use ?: and default values use = after the type.

5fill in blank

hard

Fill all three blanks to create a function with two optional parameters and default values.

Typescript

function createUser(name[1] string[2] = 'Anonymous', age[3] number[4] = 18) {
  console.log(`Name: ${name}, Age: ${age}`);
}

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

A?:

B=

C ?=

D:

Attempts:

3 left

💡 Hint

Common Mistakes

Using ?= which is invalid.

Forgetting the question mark for optional parameters.

Placing = before the type.

Optional parameters use ?: and default values use = after the type. Both parameters are optional with defaults.

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