Void type for functions in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a function that returns nothing.

Typescript

function greet(): [1] {
  console.log("Hello!");
}

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

Avoid

Bboolean

Cnumber

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' or 'number' as return type when function returns nothing.

The void type means the function does not return any value.

2fill in blank

medium

Complete the code to declare a function type that returns void.

Typescript

let logMessage: () => [1];
logMessage = () => console.log("Log");

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

Aboolean

Bstring

Cnumber

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' or 'number' as return type for a function that returns nothing.

The function type () => void means it takes no arguments and returns nothing.

3fill in blank

hard

Fix the error in the function declaration to indicate it returns nothing.

Typescript

function showAlert(): [1] {
  alert("Warning!");
}

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

Astring

Bboolean

Cvoid

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving return type as boolean but returning nothing or vice versa.

The function should return void because it does not return a value.

4fill in blank

hard

Fill both blanks to declare a function type that takes a string and returns void.

Typescript

type Logger = (message: [1]) => [2];

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

Astring

Bvoid

Cnumber

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter type or return type other than void.

The function type Logger takes a string argument and returns nothing (void).

5fill in blank

hard

Fill all three blanks to create a function that takes a number and a string and returns void.

Typescript

function processInput(id: [1], name: [2]): [3] {
  console.log(`ID: ${id}, Name: ${name}`);
}

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

Astring

Bvoid

Cnumber

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing parameter types or using a return type other than void.

The function processInput takes a number and a string as parameters and returns nothing (void).

Practice

1. What does the void type mean when used as a function's return type in TypeScript?

easy

A. The function returns a number.

B. The function returns a string.

C. The function does not return any value.

D. The function returns an object.

Solution

1. Step 1: Understand the meaning of void in TypeScript

   The void type indicates that a function does not return any value.

2. Step 2: Compare with other return types

   Other return types like number, string, or object mean the function returns those specific types, but void means no return value.

3. Final Answer:

   The function does not return any value. -> Option C

4. Quick Check:

   void means no return value [OK]

Hint: Void means no return value from the function [OK]

Common Mistakes:

• Thinking void means returning undefined explicitly
• Confusing void with any or unknown types
• Assuming void functions return null

2. Which of the following is the correct syntax to declare a function named logMessage that returns void in TypeScript?

easy

A. function logMessage(): void { console.log('Hello'); }

B. function logMessage(): string { console.log('Hello'); }

C. function logMessage(): number { console.log('Hello'); }

D. function logMessage() { return 'Hello'; }

Solution

1. Step 1: Check the function return type declaration

   The function logMessage should explicitly declare : void to indicate no return value.

2. Step 2: Verify the function body matches the return type

   The function only logs a message and does not return anything, so : void is correct.

3. Final Answer:

   function logMessage(): void { console.log('Hello'); } -> Option A

4. Quick Check:

   Use : void for no return value [OK]

Hint: Void functions have : void after parentheses [OK]

Common Mistakes:

• Omitting the return type annotation
• Using wrong return types like string or number
• Returning a value inside a void function

3. What will be the output of this TypeScript code?

function printSum(a: number, b: number): void {
  console.log(a + b);
}

const result = printSum(3, 4);
console.log(result);

medium

A. 7 and then undefined

B. 7 and then 7

C. undefined and then 7

D. undefined and then undefined

Solution

1. Step 1: Understand the function behavior

   The function printSum logs the sum of a and b but returns nothing (void).

2. Step 2: Analyze the output of the console logs

   Calling printSum(3, 4) prints 7. The variable result stores the return value, which is undefined because the function returns void. Then console.log(result) prints undefined.

3. Final Answer:

   7 and then undefined -> Option A

4. Quick Check:

   Void functions return undefined [OK]

Hint: Void functions return undefined, not a value [OK]

Common Mistakes:

• Assuming void functions return the logged value
• Expecting the function to return the sum
• Confusing console output with return value

4. Identify the error in this TypeScript function declaration:

function showMessage(): void {
  return 'Hello';
}

medium

A. No error, this is valid

B. Missing return type annotation

C. Function should not have a body

D. Function returns a string but declared as void

Solution

1. Step 1: Check the declared return type

   The function declares its return type as void, meaning it should not return any value.

2. Step 2: Analyze the return statement

   The function returns the string 'Hello', which conflicts with the void return type.

3. Final Answer:

   Function returns a string but declared as void -> Option D

4. Quick Check:

   Void functions must not return a value [OK]

Hint: Void means no return; returning value causes error [OK]

Common Mistakes:

• Returning a value inside a void function
• Ignoring TypeScript errors on return type mismatch
• Confusing void with undefined return

5. You want to write a function processData that logs a message and optionally returns a boolean indicating success. How should you declare the return type to use void correctly?

hard

A. function processData(flag: boolean): string | boolean { if(flag) return true; else console.log('No flag'); }

B. function processData(flag: boolean): boolean | void { if(flag) return true; else console.log('No flag'); }

C. function processData(flag: boolean): void { if(flag) return true; else console.log('No flag'); }

D. function processData(flag: boolean): boolean { if(flag) return true; else console.log('No flag'); }

Solution

1. Step 1: Understand the function behavior

   The function sometimes returns true (a boolean) and sometimes only logs a message without returning anything.

2. Step 2: Choose the correct return type

   Since the function can return a boolean or nothing, the return type should be a union: boolean | void. This allows both returning a boolean and returning nothing.

3. Final Answer:

   function processData(flag: boolean): boolean | void { if(flag) return true; else console.log('No flag'); } -> Option B

4. Quick Check:

   Use union type boolean | void for optional return [OK]

Hint: Use union type boolean | void for optional return values [OK]

Common Mistakes:

• Using only void when function sometimes returns a value
• Using only boolean and missing void case
• Returning value in void-only function causing errors