JavaScript is an object-based programming language used to design interactive web pages, making it indispensable in web development. In recent years, as web applications have become more complex, the use of callback functions has become a popular way to handle asynchronous code. In this article, I’ll explain what a JavaScript callback function is, how it works, and when to use it.

What are Callback Functions?

Callback functions are functions that are executed within another function. They are commonly used in asynchronous programming, where one process needs to wait for another to finish before continuing. In JavaScript, a callback function is usually a function that is passed as an argument to another function.

For example, let’s assume we have a function fetchData that fetches some data from a server as follows:

function fetchData(url, callback) {
let xhr = new XMLHttpRequest();“GET”, url);
xhr.onreadystatechange = function() {
if (xhr.readyState === XMLHttpRequest.DONE) {
if (xhr.status === 200) {

Here, `fetchData` takes two arguments: the URL of the data to fetch and the callback function. The function makes an HTTP request and then waits until the response is received. Once the response is received, `fetchData` calls the callback function with the response as an argument.

How Do Callback Functions Work?

To understand how callback functions work, let’s consider an example where we want to calculate the sum of two numbers using a callback function.

function addNumbers(num1, num2, callback) {
let sum = num1 + num2;

This function takes three arguments: two numbers, num1 and num2, and a callback function. The function calculates the sum of the two numbers and passes it to the callback function. We can use this function as follows:

function showSum(result) {

addNumbers(5, 3, showSum); // Output: 8

Here, we pass the `showSum` function as a callback function to `addNumbers`. The callback function `showSum` is executed once the result is calculated.

When to Use Callback Functions?

In JavaScript, callback functions are widely used in asynchronous programming because they can handle tasks that take an unknown amount of time to complete. For example, when fetching data from a server, it may take a few seconds or even longer to get a response. In such a scenario, using a callback function can help avoid blocking the code and waiting for the response. The code can continue executing other tasks while waiting for the response to arrive.


Q: What is the difference between synchronous and asynchronous programming?
A: In synchronous programming, a process waits for another to finish before continuing. In contrast, asynchronous programming allows a program to perform other tasks while waiting for a time-consuming operation to complete.

Q: Are callback functions the only way to handle asynchronous code in JavaScript?
A: No, there are other ways to handle asynchronous code in JavaScript, such as Promises and async/await.

Q: Can a callback function be executed multiple times?
A: Yes, a callback function can be executed multiple times if it is called from within a loop or a function that is called repeatedly.


Callback functions are an essential part of JavaScript programming, especially in asynchronous programming. They enable developers to write code that can handle tasks that take an unknown amount of time to complete, such as retrieving data from a server. By passing callback functions as arguments to other functions, developers can manage asynchronous code efficiently.

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