Type alias for functions in Typescript - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a type alias named Greet for a function that takes a string and returns a string.

Typescript

type Greet = (name: string) => [1];

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

Aboolean

Bnumber

Cvoid

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'number' or 'void' as the return type instead of 'string'.

Forgetting to specify the return type.

The function type alias should specify the return type after the arrow. Since the function returns a string, the correct return type is 'string'.

2fill in blank

medium

Complete the code to declare a variable sayHello with the type alias Greet.

Typescript

let sayHello: [1];

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

AFunction

BGreet

Cstring

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' as the type instead of the function type alias.

Using 'Function' which is too general.

To use the type alias for a function, assign the variable the type 'Greet'.

3fill in blank

hard

Fix the error in the function implementation to match the Greet type alias.

Typescript

sayHello = function(name: string): [1] {
  return "Hello, " + name;
};

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

Astring

Bvoid

Cnumber

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'void' or 'number' as the return type causing type errors.

Omitting the return type.

The function must return a string to match the 'Greet' type alias.

4fill in blank

hard

Fill both blanks to define a type alias MathOp for a function taking two numbers and returning a number.

Typescript

type MathOp = (a: [1], b: [2]) => number;

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

Anumber

Bstring

Cboolean

Dany

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'string' or 'boolean' as parameter types.

Mixing different types for parameters.

Both parameters should be of type 'number' to match the function signature.

5fill in blank

hard

Fill all three blanks to create a variable multiply of type MathOp that multiplies two numbers.

Typescript

const multiply: [1] = (x: [2], y: [3]) => x * y;

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

AMathOp

Bnumber

Cstring

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong types for parameters or variable type.

Forgetting to use the type alias for the variable.

The variable 'multiply' must have type 'MathOp'. The parameters 'x' and 'y' must be numbers.

Practice

1.

What is the main purpose of using a type alias for functions in TypeScript?

easy

A. To make the function run faster

B. To execute the function automatically

C. To convert a function into a class

D. To give a simple name to a function's input and output types

Solution

1. Step 1: Understand what a type alias does

   A type alias gives a new name to a type, including function types, making code easier to read.

2. Step 2: Identify the purpose for functions

   For functions, it means naming the input and output types so you don't repeat them everywhere.

3. Final Answer:

   To give a simple name to a function's input and output types -> Option D

4. Quick Check:

   Type alias simplifies function types = A [OK]

Hint: Type alias names function types to avoid repetition [OK]

Common Mistakes:

• Thinking type alias runs the function
• Confusing type alias with classes
• Believing it improves runtime speed

2.

Which of the following is the correct syntax to create a type alias MyFunc for a function that takes a string and returns a number?

type MyFunc = ?

easy

A. string => number

B. function(string): number

C. (input: string) => number

D. (string) => number

Solution

1. Step 1: Recall TypeScript function type syntax

   Function types use (parameters) => returnType format, with parameter names and types.

2. Step 2: Match the correct syntax

   (input: string) => number uses (input: string) => number, which is correct syntax for a function type alias.

3. Final Answer:

   (input: string) => number -> Option C

4. Quick Check:

   Function type alias syntax = D [OK]

Hint: Use (param: type) => returnType for function type alias [OK]

Common Mistakes:

• Omitting parameter name in type alias
• Using 'function' keyword inside type alias
• Writing parameter type without parentheses

3.

Given the type alias and function below, what is the output of console.log(add('5', '10'))?

type AddFunc = (a: number, b: number) => number;

const add: AddFunc = (a, b) => a + b;

console.log(add('5', '10'));

medium

A. Type error at compile time

B. 510

C. 15

D. NaN

Solution

1. Step 1: Check the type alias and function parameters

   The type alias expects two numbers as input and returns a number.

2. Step 2: Analyze the function call with string arguments

   Calling add('5', '10') passes strings, which conflicts with the expected number types, causing a compile-time error.

3. Final Answer:

   Type error at compile time -> Option A

4. Quick Check:

   Type mismatch causes compile error = C [OK]

Hint: Check argument types match alias parameters to avoid errors [OK]

Common Mistakes:

• Assuming strings convert automatically to numbers
• Expecting runtime error instead of compile error
• Thinking output is string concatenation

4.

Identify the error in this code using a type alias for a function:

type Greet = (name: string) => void;

const greet: Greet = (name) => {
  return "Hello, " + name;
};

medium

A. The function should not return a string because return type is void

B. The parameter name is missing

C. The type alias syntax is incorrect

D. The function should return a number

Solution

1. Step 1: Check the type alias return type

   The alias Greet specifies the function returns void, meaning no value returned.

2. Step 2: Check the function implementation

   The function returns a string "Hello, " + name, which conflicts with void return type.

3. Final Answer:

   The function should not return a string because return type is void -> Option A

4. Quick Check:

   Return type void means no return value = B [OK]

Hint: Void return means no value should be returned [OK]

Common Mistakes:

• Returning a value when return type is void
• Ignoring return type mismatch
• Confusing void with undefined

5.

You want to create a type alias FilterFunc for a function that takes an array of numbers and a callback function. The callback takes a number and returns a boolean. The FilterFunc returns a new array of numbers filtered by the callback. Which is the correct type alias?

hard

A. (arr: number[], callback: (num: number) => number) => boolean[]

B. (arr: number[], callback: (num: number) => boolean) => number[]

C. (arr: number[], callback: (num: number) => boolean) => boolean[]

D. (arr: number[], callback: (num: number) => boolean) => void

Solution

1. Step 1: Understand the function inputs

   The function takes an array of numbers and a callback that returns boolean for each number.

2. Step 2: Understand the function output

   The function returns a new array of numbers filtered by the callback, so return type is number[].

3. Step 3: Match the correct type alias

   (arr: number[], callback: (num: number) => boolean) => number[] matches the input and output types correctly.

4. Final Answer:

   (arr: number[], callback: (num: number) => boolean) => number[] -> Option B

5. Quick Check:

   Filter function type = A [OK]

Hint: Filter returns array of same type, callback returns boolean [OK]

Common Mistakes:

• Returning boolean[] instead of number[]
• Callback returning number instead of boolean
• Using void return type incorrectly