Callback function types in Typescript - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a callback function in TypeScript?

A callback function is a function passed as an argument to another function, which is then called inside the outer function to complete some kind of routine or action.

beginner

How do you define a callback function type in TypeScript?

You define a callback function type by specifying the parameter types and return type of the function. For example:

type Callback = (arg: string) => void;

means a function that takes a string and returns nothing.

intermediate

Why is it useful to type callback functions in TypeScript?

Typing callback functions helps catch errors early by ensuring the function passed matches the expected parameters and return type. It also improves code readability and tooling support.

intermediate

Example: How to type a function that accepts a callback with two number parameters and returns a number?

You can write: <pre>function operate(a: number, b: number, callback: (x: number, y: number) => number): number { return callback(a, b); }</pre> This means the callback takes two numbers and returns a number.

intermediate

What does this callback type mean?

(error: Error | null, result?: string) => void

It means the callback function takes two parameters: an error which can be an Error object or null, and an optional result string. The function returns nothing (void). This pattern is common in Node.js style callbacks.

What does a callback function typically do?

A Is a function that never returns

B Runs only once when the program starts

C Is passed as an argument and called later inside another function

D Is a function that only accepts numbers

A callback function is passed as an argument to another function and called later to complete an action.

How do you specify a callback type that takes a string and returns void in TypeScript?

A void => string

B (input: string) => void

C (input: number) => string

D (input: string) => string

The correct syntax is (input: string) => void, meaning it takes a string and returns nothing.

Why is typing callback functions important?

A To catch errors and improve code clarity

B To make the code run faster

C To avoid using functions

D To prevent any function from being called

Typing callbacks helps catch mistakes early and makes code easier to understand.

What does this callback type mean? (error: Error | null, result?: string) => void

A Callback with an error or null and an optional result string, returns nothing

B Callback that returns an error object

C Callback that takes no parameters

D Callback that returns a string

This is a common Node.js style callback with error-first parameter and optional result.

How would you type a function parameter that is a callback taking two numbers and returning a number?

A () => number

B (x: string, y: string) => number

C (x: number) => void

D (x: number, y: number) => number

The callback takes two numbers and returns a number, so the type is (x: number, y: number) => number.

Explain what a callback function type is and why it is useful in TypeScript.

Think about how functions can be passed as arguments and how TypeScript helps with safety.

Describe how you would type a function that accepts a callback with two number inputs and returns a number.

Focus on the function signature and the callback parameter type.