Callback function types in Typescript

Introduction

Callback function types help us tell TypeScript what kind of function we expect to use later. This keeps our code safe and clear.

When you want to run a function after another function finishes.

When you pass a function as an argument to another function.

When you want to handle events like button clicks.

When you want to customize behavior by giving a function to another function.

When you want TypeScript to check that your callback functions have the right inputs and outputs.

Syntax

Typescript

function example(callback: (param: Type) => ReturnType): void {
  // function body
}

The part (param: Type) => ReturnType describes the callback's input and output.

You can have multiple parameters in the callback by separating them with commas.

Examples

This function expects a callback that takes a string and returns nothing.

Typescript

function greet(callback: (name: string) => void) {
  callback('Alice');
}

This function takes two numbers and a callback that combines them and returns a number.

Typescript

function calculate(
  a: number,
  b: number,
  callback: (x: number, y: number) => number
): number {
  return callback(a, b);
}

Using a type alias for a callback that handles error and result.

Typescript

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

function fetchData(callback: Callback) {
  // simulate success
  callback(null, 'Data loaded');
}

Sample Program

This program processes each number in an array and calls the callback to print it.

Typescript

function processNumbers(
  numbers: number[],
  callback: (num: number) => void
): void {
  for (const num of numbers) {
    callback(num);
  }
}

processNumbers([1, 2, 3], (n) => {
  console.log(`Number is: ${n}`);
});

Important Notes

Always specify the types of parameters and return values in your callback to avoid mistakes.

Callbacks can be anonymous functions or named functions.

Using type aliases for callbacks can make your code cleaner and easier to read.

Summary

Callback function types describe what inputs and outputs a callback function should have.

They help TypeScript check your code and prevent errors.

You use them when passing functions as arguments to other functions.

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