Higher-order function types in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a higher-order function in TypeScript?

A higher-order function is a function that takes another function as an argument or returns a function as its result.

beginner

How do you type a function that takes another function as a parameter in TypeScript?

You specify the parameter as a function type, for example: <br><code>function example(fn: (x: number) => string): void { ... }</code> means <code>fn</code> is a function taking a number and returning a string.

beginner

How do you type a function that returns another function in TypeScript?

You specify the return type as a function type, for example: <br><code>function create(): (x: number) => string { return x => x.toString(); }</code> means <code>create</code> returns a function that takes a number and returns a string.

intermediate

What is the type of a function that takes a function and returns a function?

It is a higher-order function type, for example: <br>(fn: (a: number) => string) => (b: string) => number means it takes a function from number to string and returns a function from string to number.

intermediate

Why are higher-order function types useful in TypeScript?

They help describe complex function behaviors clearly, enable better type safety, and improve code readability by explicitly showing how functions interact with other functions.

Which of the following is a correct type for a function that takes a function from number to string as a parameter?

A number => (x: string) => void

B (fn: string) => number

C (fn: (x: number) => string) => void

D (x: number) => string

Option C correctly types a function that takes another function (from number to string) as a parameter.

How do you type a function that returns a function taking a boolean and returning a number?

A () => (flag: boolean) => number

B (flag: boolean) => number

C () => boolean => number

D (flag: boolean) => () => number

Option A correctly shows a function returning another function that takes a boolean and returns a number.

What does this type mean? (fn: () => void) => () => void

A A function that takes a number and returns a string

B A function that returns void

C A function that takes no parameters and returns void

D A function that takes a function with no parameters and no return, and returns a function with no parameters and no return

This type describes a higher-order function taking a function and returning a function, both with no parameters and no return value.

Which is NOT a characteristic of higher-order functions?

A They can take functions as arguments

B They always return numbers

C They can return functions

D They help create reusable code

Higher-order functions do not always return numbers; they can return any type, including functions.

Why is typing higher-order functions important in TypeScript?

A To ensure functions are used correctly and catch errors early

B To make code run faster

C To avoid using functions

D To make code shorter

Typing higher-order functions helps catch mistakes early and ensures correct usage of functions.

Explain what a higher-order function type is and give an example in TypeScript.

Think about functions that work with other functions as inputs or outputs.

Describe how you would type a function that takes a callback function and returns a new function in TypeScript.

Focus on the input and output being functions.