Type alias for functions in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a type alias for functions in TypeScript?

A type alias for functions lets you give a name to a function type, making it easier to reuse and understand the function's input and output types.

beginner

How do you define a type alias for a function that takes two numbers and returns a number?

You write: type Operation = (a: number, b: number) => number; This means Operation is a function type taking two numbers and returning a number.

beginner

Why use a type alias for functions instead of writing the function type inline?

Using a type alias makes your code cleaner and easier to read. It also helps when you use the same function type in many places, so you only write it once.

intermediate

Can a type alias for a function include optional parameters?

Yes! You can mark parameters as optional with a question mark. For example: type Greet = (name?: string) => void; means the function can be called with or without a name.

intermediate

Show an example of a type alias for a function that returns a Promise of a string.

Example: type AsyncString = () => Promise<string>; This means the function returns a Promise that will give a string later.

How do you define a type alias for a function that takes a string and returns a boolean?

A type Check = string => boolean;

B type Check = (input: string) => boolean;

C type Check = (string) => boolean;

D type Check = (input: string) boolean;

Option B correctly defines a function type alias with parameter name and type, and return type.

What does this type alias mean? type Fn = () => void;

A A function that takes no arguments and returns nothing.

B A function that takes no arguments and returns a number.

C A function that takes one argument and returns void.

D A variable that stores a function.

The empty parentheses mean no arguments, and 'void' means no return value.

Can a type alias for a function include multiple parameters?

A Yes, by listing parameters inside parentheses.

B No, only one parameter is allowed.

C Yes, but only if they are all the same type.

D No, functions can't have parameters in type aliases.

You list parameters separated by commas inside parentheses with their types.

What is the benefit of using a type alias for functions?

A It removes the need to type parameters.

B It makes the function run faster.

C It makes code easier to read and reuse.

D It changes the function's behavior.

Type aliases improve readability and reuse but do not affect runtime behavior.

How do you mark a parameter as optional in a function type alias?

A Use parentheses around the parameter.

B Use an exclamation mark after the parameter name.

C Write 'optional' before the parameter.

D Add a question mark after the parameter name.

A question mark after the parameter name marks it as optional.

Explain how to create and use a type alias for a function that adds two numbers.

Think about naming the function type and how to call a function with that type.

Describe the advantages of using type aliases for functions in TypeScript projects.

Consider how type aliases help when many functions share the same shape.

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