Asynchronous Operations in JavaScript: Callbacks, Promises, and Async/Await

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Asynchronous Operations in JavaScript: Callbacks, Promises, and Async/Await

Callbacks, Promises, and Async/Await in JavaScript

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In JavaScript, asynchronous operation is a crucial concept that we often come across while fetching data from a server, reading files, or responding to user interactions. Tasks don't execute synchronously in such cases. To manage asynchronous code efficiently, JavaScript offers various techniques, such as callbacks, promises, and async/await.

What is a Callback?

A callback is a function passed as an argument to another function, executed after that function completes its task. It's a way to specify what should happen once a certain task is finished. Callbacks are commonly used in asynchronous operations like reading files, making network requests, and handling events.

Why Use Callbacks?

JavaScript is single-threaded, which means it can only execute one operation at a time. Asynchronous operations, like fetching data from a server or reading a file, can take time to complete. Rather than blocking the entire program while waiting for these operations to finish, we use callbacks to continue with other tasks and then execute the callback when the operation is done.

Example 1: Synchronous Callback

Let's start with a simple synchronous callback example. Here, we'll define a function that takes two numbers and a callback function to perform an operation on those numbers.

function calculate(a, b, callback) {
  const result = callback(a, b);
  return result;

function add(x, y) {
  return x + y;

function subtract(x, y) {
  return x - y;

const sum = calculate(5, 3, add);
console.log("Sum:", sum);

const difference = calculate(5, 3, subtract);
console.log("Difference:", difference);

In this example, the calculate function takes two numbers and a callback function as arguments. It then invokes the callback function with those numbers and returns the result. We use it to calculate both the sum and difference of two numbers by passing the add and subtract functions as callbacks.

Example 2: Asynchronous Callback

Now, let's look at an asynchronous callback example using the setTimeout function to simulate a delayed operation:

function fetchData(callback) {
  setTimeout(function () {
    const data = "This is some data from a server.";
  }, 2000); // Simulate a 2-second delay

function process(data) {
  console.log("Processing data:", data);

console.log("Fetching data...");
console.log("Data fetched.");

In this example, the fetchData function simulates fetching data from a server with a 2-second delay. It takes a callback function (process) and calls it with the fetched data when the operation is complete. While waiting for the data to be fetched, the program can continue with other tasks.

Example 3: Callback Hell (Callback Pyramid)

Callback hell, also known as the "pyramid of doom" or "callback pyramid," is a situation in JavaScript where we have multiple nested callbacks within one another. This occurs when we have asynchronous operations that depend on the results of previous asynchronous operations. As more nested callbacks are added, the code becomes difficult to read and maintain. This can lead to code that is challenging to understand and prone to errors.

Let's explore callback hell in more detail with examples.

Example: Nested setTimeouts

In this example, we have three asynchronous operations using setTimeout, and each operation depends on the result of the previous one.


setTimeout(function () {
  console.log("First step completed");

  setTimeout(function () {
    console.log("Second step completed");

    setTimeout(function () {
      console.log("Third step completed");

      // More nested callbacks can make the code even worse
    }, 1000);
  }, 1000);
}, 1000);

As you can see, as more steps are added, the code indentation and nesting levels increase, making it harder to follow the flow of the program.

Example: Nested Calls (Callback Pyramid)

In this example, we simulate making multiple requests with callbacks, where each request depends on the result of the previous one.

function fetchData1(callback) {
  setTimeout(function () {
    console.log("Data 1 fetched");
  }, 1000);

function fetchData2(callback) {
  setTimeout(function () {
    console.log("Data 2 fetched");
  }, 1000);

function fetchData3() {
  setTimeout(function () {
    console.log("Data 3 fetched");
  }, 1000);

console.log("Fetching data...");

fetchData1(function () {
  fetchData2(function () {

In this scenario, we have three requests, and each callback depends on the completion of the previous request. The nesting of callbacks can quickly become challenging to manage as the number of dependent asynchronous operations increases.

Problems with Callback Hell:

  1. Readability: The code becomes difficult to read and understand due to excessive nesting and indentation.

  2. Maintenance: Making changes or debugging in callback hell can be a nightmare because it's easy to lose track of the flow.

  3. Error Handling: Error handling becomes cumbersome, as we need to add error checks and handling in each callback.

Using nested callbacks extensively, also known as "callback hell" or "pyramid of doom," is generally not considered a good practice in modern JavaScript development. While callbacks are a fundamental concept for handling asynchronous operations. Excessive nesting of callbacks can lead to code that is difficult to read, understand, and maintain. It can also make error handling more complex.

Here's a brief overview of best practices when working with callbacks:

  1. Avoid Excessive Nesting: Refrain from deeply nesting callbacks. Instead, use named functions for callbacks to make the code more readable and maintainable.

  2. Error Handling: Ensure that we handle errors properly within callbacks. Use try-catch blocks or error-first callback patterns to manage errors effectively.

  3. Modularization: Break down the code into smaller, reusable functions and use callbacks to compose them. This promotes a more modular and testable codebase.


Promises are a better alternative to callbacks for managing asynchronous operations in JavaScript. Promises provide a more structured way to handle asynchronous code and improve code readability.

It serves as a placeholder or a container for the eventual result of an asynchronous operation. A promise represents a value that may not be available yet but will be at some point in the future, or it might represent an operation that could succeed or fail. Promises provide a way to work with asynchronous code in a more structured and predictable manner.

Here's how we typically use a promise to get data in an async call:

  1. Creating a Promise: We create a new promise object, which takes a function as an argument. This function has two parameters: resolve and reject. Inside this function, it performs our async operation, and when it's done, it calls either resolve with the result or reject with an error if something goes wrong.
const fetchData = () => {
  return new Promise((resolve, reject) => {
    // fetching data from a server
    setTimeout(() => {
      const data = "This is some data from an async call.";
      // Resolve the promise with the data
      // Or reject it with an error if something went wrong
      // reject("An error occurred during the async call.");
    }, 2000);
  1. Using the Promise: Once we have a promise object, we can use it by calling methods like .then() to specify what should happen when the promise resolves successfully, or .catch() to handle errors if the promise is rejected.
  .then((data) => {
    console.log("Data fetched:", data);
    // Do something with the fetched data
  .catch((error) => {
    console.error("Error:", error);
    // Handle the error

In this example:

  • When the fetchData promise resolves, the .then() callback is executed, allowing us to work with the fetched data.

  • If an error occurs during the async call, the .catch() callback is executed to handle the error.

Hence, promises provide a structured way to work with async code and ensure that the code doesn't block while waiting for results from async operations.


Async/await is a further improvement in managing asynchronous operations and is built on top of promises. It offers a more synchronous-looking code structure while preserving non-blocking behavior. Here's an example:

async function fetchData() {
  try {
    const response = await fetch('<>');
    const data = await response.json();
    console.log("Data fetched:", data);
    // Perform additional operations with the data
  } catch (error) {
    console.error("Error:", error);

console.log("Fetching data...");


In this example:

  • The async keyword is used to define an asynchronous function.

  • Inside the function, await is used to pause execution until the promise is resolved or rejected, making the code appear synchronous.

Difference between Promise Chaining and Async/Await

Here is a crucial difference between promise chaining using .then() and using async/await in JavaScript when it comes to the execution flow. Promise chaining does not block the rest of the code execution, while await does. This is because promise chaining uses callbacks, which are executed asynchronously. When a promise is resolved or rejected, the callback function is added to the event queue. The event loop then executes the callback function when it gets to it. This means that the rest of the code continues to execute while the promise is pending.

Await, on the other hand, blocks the rest of the code execution until the promise is resolved or rejected. This is because await waits for the promise to resolve or reject before it continues executing the code.

Let's break down the difference between promise chaining and async/await using examples in simpler terms.

  1. Promise Chaining:

    In the first example, with promise chaining:

     function foo() {
       return new Promise(resolve => {
         setTimeout(() => resolve('Hello, world!'), 1000);
     function bar() {
       foo().then(message => console.log(message));

    Here's what's happening:

    • bar() calls foo(), which returns a promise.

    • Instead of waiting for the promise to finish, bar() continues to execute immediately.

    • It prints "Rwitesh" to the console right away.

    • The promise resolves after a delay of 1 second, and then it prints "Hello, world!" to the console.

In summary, promise chaining allows the code to keep running while the promise is processing in the background. It doesn't block the rest of the code.

  1. async/await:

    In the second example, with async/await:

     async function baz() {
       const message = await foo();

    Here's what's happening:

    • baz() is an asynchronous function that uses await with foo().

    • When it encounters await foo(), it stops and waits for foo() to complete.

    • Only after foo() is done (in 1 second), it continues and prints "Hello, world!" and then "Rwitesh" to the console.

In summary, async/await pauses the execution of the function until the awaited promise is resolved. It waits for the promise to finish before moving on.

So, promise chaining allows for non-blocking code execution, while async/await making the code wait for a promise to complete before moving forward.


In conclusion, when dealing with asynchronous tasks in JavaScript, it's essential to choose the right tool for the job. Callbacks, while fundamental, can lead to complex and hard-to-read code when deeply nested. Promises and async/await are more modern and structured approaches to handling asynchronous operations.

Promises provide a clear way to work with asynchronous code. It allows us to define what happens when an operation succeeds or fails. Async/await takes this a step further by making our code look and feel synchronous while maintaining its non-blocking nature.

So, when working with asynchronous code in JavaScript, we should consider using promises or async/await to keep the code readable, maintainable, and efficient, and avoid falling into the callback hell trap.