Callback function types in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a callback type that takes a number and returns void.

Typescript

type Callback = (num: number) => [1];

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

Astring

Bnumber

Cvoid

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using a return type other than void when the function returns nothing.

The callback function returns void because it does not return any value.

2fill in blank

medium

Complete the code to define a function that accepts a callback with a string parameter.

Typescript

function greet(callback: (message: [1]) => void) {
  callback('Hello!');
}

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

Anumber

Bstring

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using a type that does not match the argument passed to the callback.

The callback expects a string parameter because it receives a greeting message.

3fill in blank

hard

Fix the error in the callback type to accept two parameters: a number and a string.

Typescript

type MyCallback = ([1]) => void;

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

Anum: number, text: string

Bnum: string

Cnum: number

Dtext: string

Attempts:

3 left

💡 Hint

Common Mistakes

Providing only one parameter or wrong parameter types.

The callback must accept two parameters: a number and a string, separated by a comma.

4fill in blank

hard

Fill both blanks to declare a callback type that takes a boolean and returns a number.

Typescript

type BoolToNum = (flag: [1]) => [2];

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

Aboolean

Bstring

Cnumber

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up parameter and return types.

The callback takes a boolean and returns a number.

5fill in blank

hard

Fill all three blanks to define a function that accepts a callback with two parameters and returns void.

Typescript

function processData(callback: ([1]: number, [2]: string) => [3]) {
  callback(42, 'data');
}

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

Aid

Bname

Cvoid

Dnumber

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter names or return type.

The callback parameters are named id and name, and the return type is void.

Practice

1. What does a callback function type in TypeScript specify?

easy

A. The input parameters and return type of the callback function

B. The variable names used inside the callback function

C. The order in which functions are called

D. The number of times a function is called

Solution

1. Step 1: Understand callback function types

   A callback function type defines what inputs (parameters) the callback expects and what it returns.

2. Step 2: Identify what is specified

   It does not specify variable names, call order, or call count, only input and output types.

3. Final Answer:

   The input parameters and return type of the callback function -> Option A

4. Quick Check:

   Callback type = inputs and output [OK]

Hint: Callback types describe inputs and outputs only [OK]

Common Mistakes:

• Confusing callback type with variable names
• Thinking callback type controls call order
• Assuming callback type limits call count

2. Which of the following is the correct TypeScript syntax to declare a callback type that takes a number and returns void?

easy

A. type Callback = (input: number) => void;

B. type Callback = (number) => void;

C. type Callback = (input: number) => number;

D. type Callback = void (input: number);

Solution

1. Step 1: Check correct syntax for callback type

   In TypeScript, a function type is declared as (parameters) => returnType. Parameters must have names and types.

2. Step 2: Match the correct option

   type Callback = (input: number) => void; correctly declares a function taking a number named 'input' and returning void.

3. Final Answer:

   type Callback = (input: number) => void; -> Option A

4. Quick Check:

   Correct syntax = type Callback = (input: number) => void; [OK]

Hint: Use (param: type) => returnType syntax [OK]

Common Mistakes:

• Omitting parameter name
• Wrong return type
• Incorrect arrow function syntax

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

type Callback = (x: number) => number;
function applyCallback(n: number, cb: Callback) {
  return cb(n) + 1;
}
const result = applyCallback(5, x => x * 2);
console.log(result);

medium

A. 10

B. 11

C. 6

D. Error

Solution

1. Step 1: Understand the callback and function call

   The callback multiplies input by 2. applyCallback calls cb(5), which is 5*2=10, then adds 1.

2. Step 2: Calculate the final result

   10 + 1 = 11, so console.log prints 11.

3. Final Answer:

   11 -> Option B

4. Quick Check:

   5*2+1 = 11 [OK]

Hint: Calculate callback result then add 1 [OK]

Common Mistakes:

• Forgetting to add 1 after callback
• Confusing callback return with input
• Assuming callback returns void

4. Identify the error in this TypeScript code snippet:

type Callback = (msg: string) => void;
function greet(cb: Callback) {
  cb(123);
}

medium

A. Function greet should not accept a callback

B. Callback should return a number, not void

C. Callback expects a string, but a number is passed

D. No error, code is correct

Solution

1. Step 1: Check callback type and usage

   Callback expects a string parameter, but greet calls cb with 123 (a number).

2. Step 2: Identify type mismatch error

   Passing a number where a string is expected causes a type error.

3. Final Answer:

   Callback expects a string, but a number is passed -> Option C

4. Quick Check:

   Type mismatch in callback argument [OK]

Hint: Match callback parameter types exactly [OK]

Common Mistakes:

• Passing wrong argument type to callback
• Ignoring callback return type
• Assuming no error on type mismatch

5. You want to create a function processItems that takes an array of numbers and a callback. The callback should receive each number and return a string.
Which is the correct callback type and function signature to ensure type safety?

hard

A. type Callback = (item: string) => number;
function processItems(items: string[], cb: Callback): number[];

B. type Callback = () => string;
function processItems(items: number[], cb: Callback): string[];

C. type Callback = (item: number) => void;
function processItems(items: number[], cb: Callback): void;

D. type Callback = (item: number) => string;
function processItems(items: number[], cb: Callback): string[];

Solution

1. Step 1: Define callback input and output types

   The callback should take a number and return a string, so type Callback = (item: number) => string.

2. Step 2: Define function signature matching callback and array

   processItems takes number[] and callback, returns string[] because it maps numbers to strings.

3. Final Answer:

   type Callback = (item: number) => string; function processItems(items: number[], cb: Callback): string[]; -> Option D

4. Quick Check:

   Callback input number, output string [OK]

Hint: Match callback input/output with function parameters/return [OK]

Common Mistakes:

• Mismatching callback parameter types
• Wrong return type in callback or function
• Ignoring array element types