Kriza Educa

Kriza Educa


Understand the concept of the Callback, Promise and Async/Await in JavaScript

Callback vs Promises vs Async Await

This blog explains the fundamental concepts that JavaScript relies on to handle asynchronous operations. These concepts include Callback functionsPromises and the use of Async, and Await to handle deferred operations in JavaScript.

So before we decode the comparison between the three, let’s get a brief understanding of synchronous (blocking) and asynchronous(non-blocking).

Difference Between Sync and Async

To make it easy to understand, let’s take a live example which probably will explain the difference between asynchronous and synchronous.

Imagine we go to a restaurant, a waiter comes to a table, takes your order and gives it to the kitchen. Then they move on to the server on another table, while the chef is preparing the meal So the same person can serve the many different tables. The table has to wait for the chef to cook one meal before they serve another table. This is what we called asynchronous or non-blocking architecture. Here the waiter is like a thread allocated to handle requests. So a single thread is used to handle multiple requests.

In contrast to non-blocking or asynchronous architecture, we have blocking or synchronous architecture. Let’s see how that works. So back to the restaurant example, imagine you go to another restaurant and in this restaurant, a waiter is allocated to you. He takes your order and gives it to the kitchen. Now he is sitting in the kitchen waiting for the chef to prepare your meal and this time he is not doing anything else he is just waiting for he is not going to take any order from another table until your meal is ready. This is what we called synchronous or blocking architecture.

Now, the first restaurant example represents an asynchronous process because you did not have to wait, the waiter takes the order from one table and goes to the next table to take the order. While the second example restaurant represents a synchronous operation because you had to wait until the resource (waiter in this case) can proceed with you. This is the single, most fundamental difference between sync and async processes.

One important thing to keep in mind is that the single-threaded event handling systems are usually implemented using an event or message queue. So when a program is being executed synchronously, the thread will wait until the first statement is finished to jump to the second one, while in asynchronous execution, even if the first one was not completed, the execution will continue.

There are different ways to handle the async code. Those are callbacks, promises, and async/await.


In JavaScript, functions are objects. So we can pass objects to functions as parameters.

We can also pass functions as parameters to other functions and call them inside the outer functions. So callback is a function that is passed to another function. When the first function is done, it will run the second function.

Let’s take an example of callback function:

function printString(){
   setTimeout(function()  { console.log("Jacob"); }, 300); 


If that were sync code, we would have encountered the following output.


But the setTimeout is an async function then the output of the above code will be:


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).

In other words, the message function is being called after something happened (after 3 seconds passed for this example), but not before. So the callback is the function that is passed as the argument to setTimeout.

Callback as an Arrow Function:

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

function printString(){
   setTimeout(() =>  { console.log("Jacob"); }, 300); 


The output will be the same as above.

The problem with callbacks is it creates something called “Callback Hell.” Basically, you start nesting functions within functions within functions, and it starts to get really hard to read the code. So in this situation Promises came to handle the nested callback in a better way.


A promise in JavaScript is similar to a promise in real life. When we make a promise in real life, it is a guarantee that we are going to do something in the future. Because promises can only be made for the future.

A promise has two possible outcomes: it will either be kept when the time comes, or it won’t.

This is also the same for promises in JavaScript. When we define a promise in JavaScript, it will be resolved when the time comes, or it will get rejected. It sounds like the IF condition. But there are huge differences between them.

A promise is used to handle the asynchronous result of an operation. JavaScript is designed to not wait for an asynchronous block of code to completely execute before other synchronous parts of the code can run. With Promises, we can defer the execution of a code block until an async request is completed. This way, other operations can keep running without interruption.

States of Promises:

First of all, a Promise is an object. There are 3 states of the Promise object:

  • Pending: Initial State, before the Promise succeeds or fails.
  • Resolved: Completed Promise
  • Rejected: Failed Promise, throw an error

For example, when we request data from the server by using a Promise, it will be in pending mode until we receive our data.

If we achieve to get the information from the server, the Promise will be resolved successfully. But if we don’t get the information, then the Promise will be in the rejected state.

Creating a Promise:

Firstly, we use a constructor to create a Promise object. The promise has two parameters, one for success (resolve) and one for fail (reject):

const myPromise = new Promise((resolve, reject) => {  
    // condition

Let’s create a promise:

const myFirstPromise = new Promise((resolve, reject) => { 
    const condition = true;   
    if(condition) {
             resolve("Promise is resolved!"); // fulfilled
        }, 300);
    } else {    
        reject('Promise is rejected!');  

In the above Promise If Condition is true, resolve the promise returning the “Promise is resolved ”, else return an error “Promise is rejected”. Now we have created our first Promise, Now let’s use it.

Using Promise:

To use the above create Promise we use then() for resolve and catch() for reject.

.then((successMsg) => {
.catch((errorMsg) => { 

let’s take this a step further:

const demoPromise= function() {
  .then((successMsg) => {
      console.log("Success:" + successMsg);
  .catch((errorMsg) => { 
      console.log("Error:" + errorMsg);


In our created promise condition is “true” and we call demoPromise() then our console logs read:

Success: Promise is resolved!

So if the promise gets rejected, it will jump to the catch() method and this time we will see a different message on the console.

Error: Promise is rejected!

What is Chaining?

Sometimes we need to call multiple asynchronous requests, then after the first Promise is resolved (or rejected), a new process will start to which we can attach it directly by a method called chaining.

So we create another promise:

const helloPromise  = function() {
  return new Promise(function(resolve, reject) {
    const message = `Hi, How are you!`;


We chain this promise to our earlier “myFirstPromise” operation like so:

const demoPromise= function() {

  .then((successMsg) => {
      console.log("Success:" + successMsg);
  .catch((errorMsg) => { 
      console.log("Error:" + errorMsg);


Since our condition is true, the output to our console is:

Hi, How are you!

Once the “hello” promise is chained with .then, subsequent .then utilizes data from the previous one.


Await is basically syntactic sugar for Promises. It makes your asynchronous code look more like synchronous/procedural code, which is easier for humans to understand.

Syntax of Async and Await:

async function printMyAsync(){
  await printString("one")
  await printString("two")
  await printString("three")

You can see that we use the “async” keyword for the wrapper function printMyAsync. This lets JavaScript know that we are using async/await syntax, and is necessary if you want to use Await. This means you can’t use Await at the global level. It always needs a wrapper function. Or we can say await is only used with an async function.

The await keyword is used in an async function to ensure that all promises returned in the async function are synchronized, ie. they wait for each other. Await eliminates the use of callbacks in .then() and .catch(). In using async and await, async is prepended when returning a promise, await is prepended when calling a promise. try and catch are also used to get the rejection value of an async function.

Let’s take an example to understand the Async and Await with our demoPromise:

async function demoPromise() {
  try {
    let message = await myFirstPromise;
    let message  = await helloPromise();

  }catch((error) => { 
      console.log("Error:" + error.message);

// finally, call our async function

(async () => { 
  await myDate();


In this tutorial, we understand the concept of the callback, Promise, and Async/Await. We know how they can work with javascript asynchronous requests. Mainly they used API Request and event handling.

#api #javascript

What is GEEK

Buddha Community

Understand the concept of the Callback, Promise and Async/Await in JavaScript

Gadir Novruzov



Gadir Novruzov



Gadir Novruzov


Gadir Novruzov


Giles  Goodwin

Giles Goodwin


Understanding JavaScript: Promises, Async & Await!!


We all know the importance of promises in our life. We even have a special day dedicated to it :) But how well do we know the importance of promises in JavaScript? Well if you don’t know it yet, it’s a great time to know it because they are becoming more and more popular. So what are promises? Let’s try to understand it through an analogy.

Suppose you are a top class rapper and you haven’t released an album for a while and fans are asking for it day and night. So what you do is that you “promise” them that whenever it will be out, all of them would be notified. To get this done you give your fans a list. They can fill in their email addresses, so that when the album becomes available, all the subscribers instantly receive it. And even if something goes wrong, say a pandemic, so that you can’t release the album, they will still be notified.

Now everyone is happy: You, because the people don’t crowd you anymore, and fans, because they won’t miss any news on the album.

This is a real-life analogy for things we often have in programming:

  1. “producing code” that does something and may take time. That’s a “rapper”.
  2. “consuming code” that wants the result of the “producing code” once it’s ready. Many functions may need that result. These are the “fans”.
  3. A promise is a special JavaScript object that links the “producing code” and the “consuming code” together. In terms of our analogy: this is the “subscription list”. The “producing code” takes whatever time it needs to produce the promised result, and the “promise” makes that result available to all of the subscribed code when it’s ready.

JavaScript promises are much more complex than a simple subscription list: they have additional features and limitations. But it’s fine to begin with.

#async #promises #javascript #development #await

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

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

Code  JS

Code JS


JavaScript Fetch API Explained | Callbacks, Promises, Async Await

Learn about Callbacks, Promises, and Async Await as the JavaScript Fetch API is explained in this tutorial. You will also learn about thenables and how async / await replaces them in our JS code. The first 30 minutes covers the concepts. The last 30 minutes gives examples of retrieving data from different APIs with Fetch.

Quick Concepts outline:
Fetch API with Async / Await
(0:00) Intro
(0:29) What is a callback function?
(1:15) What is the problem with callbacks?
(3:00) JavaScript Promises have 3 states
(5:28) A promise may not return a value where you expect it to: You need to wait for a promise to resolve
(6:58) Using thenables with a promise
(20:15) An easy mistake to make with promises
(24:00) Creating an async function
(25:00) Applying await inside the function
(33:45) Example 1: Retrieving user data
(40:00) Example 2: Retrieving dad jokes
(47:00) Example 3: Posting data
(49:40) Example 4: Retrieving data with URL parameters
(54:55) Abstract it all into single responsibility functions


#async  #await  #fetch #javascript 

Niraj Kafle


The essential JavaScript concepts that you should understand

As a JavaScript developer of any level, you need to understand its foundational concepts and some of the new ideas that help us developing code. In this article, we are going to review 16 basic concepts. So without further ado, let’s get to it.

#javascript-interview #javascript-development #javascript-fundamental #javascript #javascript-tips

Javascript : Async Await in Javascript | Javascript Interview questions.

Explained in detail about

  • Async
  • Await
  • Fetching country data using Async & Await
  • How Async & Await better than promise

Dont miss to watch the video & ask your questions or doubts in comments

#javascript #async #await