Dipen Shah

Dipen Shah


JavaScript Callback Functions Tutorial with Examples

If you’re familiar with programming, you already know what functions do and how to use them. But what is a callback function? Callback functions are an important part of JavaScript and once you understand how callbacks work, you’ll become much better in JavaScript.

So in this post, I would like to help you to understand what callback functions are and how to use them in JavaScript by going through some examples.

What is a Callback Function?

In JavaScript, functions are objects. Can we pass objects to functions as parameters? Yes.

So, we can also pass functions as parameters to other functions and call them inside the outer functions. Sounds complicated? Let me show that in an example below:

function print(callback) {  

The print( ) function takes another function as a parameter and calls it inside. This is valid in JavaScript and we call it a “callback”. So a function that is passed to another function as a parameter is a callback function. But that’s not all.

Why do we need Callback Functions?

JavaScript runs code sequentially in top-down order. However, there are some cases that code runs (or must run) after something else happens and also not sequentially. This is called asynchronous programming.

Callbacks make sure that a function is not going to run before a task is completed but will run right after the task has completed. It helps us develop asynchronous JavaScript code and keeps us safe from problems and errors.

In JavaScript, the way to create a callback function is to pass it as a parameter to another function, and then to call it back right after something has happened or some task is completed. Let’s see how…

How to create a Callback

To understand what I’ve explained above, let me start with a simple example. We want to log a message to the console but it should be there after 3 seconds.

const message = function() {  
    console.log("This message is shown after 3 seconds");

setTimeout(message, 3000);

There is a built-in method in JavaScript called “setTimeout”, which calls a function or evaluates an expression after a given period of time (in milliseconds). So here, the “message” function is being called after 3 seconds have passed. (1 second = 1000 milliseconds)

In other words, the message function is being called after something happened (after 3 seconds passed for this example), but not before. So the message function is an example of a callback function.

What is an Anonymous Function?

Alternatively, we can define a function directly inside another function, instead of calling it. It will look like this:

setTimeout(function() {  
}, 3000);

As we can see, the callback function here has no name and a function definition without a name in JavaScript is called as an “anonymous function”. This does exactly the same task as the example above.

Callback as an Arrow Function

If you prefer, you can also write the same callback function as an ES6 arrow function, which is a newer type of function in JavaScript:

setTimeout(() => { 
}, 3000);

What about Events?

JavaScript is an event-driven programming language. We also use callback functions for event declarations. For example, let’s say we want users to click on a button:

<button id="callback-btn">Click here</button>

This time we will see a message on the console only when the user clicks on the button:

    .addEventListener("click", function() {    
      console.log("User has clicked on the button!");

So here we select the button first with its id, and then we add an event listener with the addEventListener method. It takes 2 parameters. The first one is its type, “click”, and the second parameter is a callback function, which logs the message when the button is clicked.

As you can see, callback functions are also used for event declarations in JavaScript.

Wrap up

Callbacks are used often in JavaScript, and I hope this post helps you understand what they actually do and how to work with them easier. The next thing you can learn is JavaScript Promises which is a similar topic.

Thank you for reading!

#javascript #web-development

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Buddha Community

JavaScript Callback Functions Tutorial with Examples
Jeromy  Lowe

Jeromy Lowe


Intro to Callback in JavaScript

During my journey of learning JavaScript and a few days in with React, I had to come to terms with callback functions. It was impossible to avoid its existence and blindly using it without understanding. To understand callback function we have to know a bit about functions. Here I want to talk about function expression, arrow function, and callback function.

What is a Callback function?

According to MDN doc:


In JavaScript, functions are objects, which means, like any other objects it can be assigned to a variable and passed as an argument to another function. In a nutshell, a callback function is a function that is provided as a parameter for other methods such as forEach method or addEventListener method and gets invoked at a certain point in time.

Passing functions as arguments

So how do we do this? Let’s see with an example below:


//whoAmI Function Declaration(Function Statement)
function whoAmI(event){

We attached the ‘click’ event listener to document with whoAmI function as a parameter that logs the tag name of the clicked target. Whenever ‘click’ happens whoAmI function will be called with an _event_ argument. We call whoAmI a callback function.

When we call a function by its name followed by ( ), we are telling the function to execute its code. When we name a function or pass a function without the ( ), the function does not execute.** The callback function doesn’t execute right away. Instead, the addEventListener method executes the function when the event occurs.**

One more thing I want to mention is because we used function declaration, we were able to call thewhoAmI function before it was declared. It’s the magic of hoisting in JS. But with function expression, it doesn’t get hoisted. So the order of writing function expressions and using them as callback would be crucial.

#callback #javascript #callback-function #function #javascript-fundamental

Vincent Lab

Vincent Lab


The Difference Between Regular Functions and Arrow Functions in JavaScript

Other then the syntactical differences. The main difference is the way the this keyword behaves? In an arrow function, the this keyword remains the same throughout the life-cycle of the function and is always bound to the value of this in the closest non-arrow parent function. Arrow functions can never be constructor functions so they can never be invoked with the new keyword. And they can never have duplicate named parameters like a regular function not using strict mode.

Here are a few code examples to show you some of the differences
this.name = "Bob";

const person = {
name: “Jon”,

<span style="color: #008000">// Regular function</span>
func1: <span style="color: #0000ff">function</span> () {
    console.log(<span style="color: #0000ff">this</span>);

<span style="color: #008000">// Arrow function</span>
func2: () =&gt; {


person.func1(); // Call the Regular function
// Output: {name:“Jon”, func1:[Function: func1], func2:[Function: func2]}

person.func2(); // Call the Arrow function
// Output: {name:“Bob”}

The new keyword with an arrow function
const person = (name) => console.log("Your name is " + name);
const bob = new person("Bob");
// Uncaught TypeError: person is not a constructor

If you want to see a visual presentation on the differences, then you can see the video below:

#arrow functions #javascript #regular functions #arrow functions vs normal functions #difference between functions and arrow functions

Terry  Tremblay

Terry Tremblay


Now Learn JavaScript Programming Language With Microsoft

icrosoft has released a new series of video tutorials on YouTube for novice programmers to get a hands-on renowned programming language — JavaScript.

This isn’t the first attempt by Microsoft to come up with video tutorials by beginner programmers. The company also has a series of YouTube tutorials on Python for beginners.

For JavaScript, Microsoft has launched a series of 51 videos as ‘Beginner’s Series to JavaScript,’ for young programmers, developers and coders who are interested in building browser applications using JavaScript. These video tutorials will also help programmers and coders to use relevant software development kits (SDKs) and JavaScript frameworks, such as Google’s Angular.

“Learning a new framework or development environment is made even more difficult when you don’t know the programming language,” stated on the Microsoft Developer channel on YouTube. “Fortunately, we’re here to help! We’ve created this series of videos to focus on the core concepts of JavaScript.”

It further stated — while the tutorials don’t cover every aspect of JavaScript, it indeed will help in building a foundation from which one can continue to grow. By the end of this series, Microsoft claims that the novice programmers will be able to work through tutorials, quick starts, books, and other resources, continuing to grow on their own.

#news #javascript #javascript tutorial #javascript tutorials #microsoft tutorials on javascript

Hugo JS

Hugo JS


What is Callback-Function in JavaScript? How it is Replaced By Promises?

Let us understand term _Callback _by an real-world example: Suppose, you are calling to your Girlfriend (If you have) and she is busy in another call then she send message to you : “I am busy right now, Call back you later.!!”. After completing her work, she calls you back and this is what call back in JavaScript as well.

In JavaScript, When a function is executing (Girlfriend is talking with someone) then after function execution is completed another function is started for execution this is call back function.

Now you are thinking that its depend upon when, where you are calling function and all function call is “Call-back function”. Image for post

Here, _printWorld() _function is executed after _printHello() _complete its execution but this is not call-back function example because _printHello() _is not Asynchronous function. Suppose, _printHello() _prints after 1 Second then _printWorld() _executes first.

Image for post

What if we want “Hello World” output when Asynchronous function is there. We can pass function as argument and calls it after _printHello() _complete its execution. Here below code snippet of how _function pass as argument _:

Image for post

Callback function can be defined as a function passed by argument and executes when one function completes its execution.

Suppose, If you have API (Application Programming Interface) to get Students Roll numbers and select one of Roll number — getting that roll number’s data and print that data. We don’t have API to get students data so we are using _setTimeout() _Async function and getting roll number after 2s, We are also selecting one of roll number manually after 2s and print Roll number’s data after 2s. This can be done by call back function.

Image for post

The program became complex and complex if we have too many things to do like Getting Students data, Selecting one of them student, get student’s roll number and get result by roll number then it become very complex. If you have any Error in this then debugging is also tedious task, this things is called “Callback Hell”, which is shape like “Pyramid Of Doom”.

To overcome with this problem, Promises is introduced in JavaScript. Promises has three states : Pending, Resolved, Reject. Promises is created by Constructor : new Promise(). It has one executor function which has two arguments (resolve, reject).

Promise object has three methods: then(), catch() & finally().

Image for post

If Promise is successfully executed then its data is transferred through resolve function and if it has error then passed through reject function.

We have implemented same task which is done using call back function in Promises and its easily understandable However it is complicated compare to callback function but when you use promises for sometimes then it’s easy to implement.

In _getRollNumber(), _resolve method’s data is caught by then() functions arguments and reject method’s data is caught by catch() function. Here In Promises, Every task has different promises because of that it is easy to debug and readable compare to call back function. You can see that there is no shape like “Pyramid of Doom” in Promises. This is how Callback function is replaced by Promises.

Thank you for reading!

This article was originally published on Medium.com

#javascript-tips #advanced-javascript #javascript #callback-function #promises

wp codevo

wp codevo


JavaScript Shopping Cart - Javascript Project for Beginners

