Altcademy - a Forbes magazine logo Best Coding Bootcamp 2023

What is callback function in JavaScript

Understanding Callback Functions in JavaScript

At some point during your coding journey, you've likely come across the term 'callback function.' This concept can feel a tad confusing, especially if you're new to programming. But don't worry, we're going to demystify it together!

So, what is a callback function in JavaScript? In essence, a callback function is a function that is passed as an argument into another function. It's then executed (or 'called back') inside the outer function.

The Analogy Way

Think of a callback function like going to a coffee shop. You place your order (calling a function), then you sit and wait until your order is ready (the function is processing). Once your order is ready, your name is called (function execution is complete), and you pick up your coffee (the callback function is executed). This is effectively how callback functions work in JavaScript.

Breaking Down the Callback Function

To understand callback functions better, let's break it down into two parts: the 'calling function' and the 'callback function.'

function callingFunction(callback){
    // Some code here...

In the code above, callingFunction is the function that receives another function (callback) as an argument. The callback() inside callingFunction is where the passed-in function gets executed.

Let's see a practical example:

function greet(name, callback){
    console.log('Hello ' + name);

function sayGoodbye(){

greet('John Doe', sayGoodbye);

In the example above, when we call greet('John Doe', sayGoodbye);, the function greet is executed first. It prints 'Hello John Doe' and then calls the sayGoodbye function (our callback function), which prints 'Goodbye!'.

Why Use Callback Functions?

Callback functions are incredibly handy because they allow our code to be more flexible and reusable. This is because we can change the function that gets executed without modifying the function that calls it. It also helps us manage asynchronous operations in JavaScript, where we need to wait for some process to complete before executing the next step.

Anonymous Callback Functions

Sometimes, instead of passing a pre-defined function into another function, we define the function right there on the spot. These are known as "anonymous functions". Here's an example:

greet('John Doe', function(){

In this case, the sayGoodbye function is replaced by an anonymous function that performs the same action.

Callback Hell and Promises

While callbacks can be very powerful, they can also lead to a situation humorously referred to as 'callback hell.' This happens when you have many nested callback functions, making the code hard to read and maintain.

            // And so on...

To avoid 'callback hell,' JavaScript introduced Promises and async/await, which help to write asynchronous code in a more manageable and readable way. But that's a topic for another day!


Imagine being a DJ at a party. You're not just playing songs; you’re also taking song requests (callbacks) from the party-goers. The party is your program, the songs are your functions, and the party-goers are other bits of code trying to interact with your functions. As the DJ, you accept the requests and play them at the right time. That's your role as a JavaScript developer handling callback functions!

Callback functions may seem a little tricky initially, but once you get the hang of them, they'll become a powerful tool in your JavaScript toolbox. They offer a fantastic way to create more flexible, interactive, and responsive web applications. So, keep practicing, keep coding, and soon enough, you'll be handling callbacks like a pro DJ handling song requests at a party!