Home » JavaScript » Promises in JavaScript: Def, Creation, Call, Pros and Cons, Behind-the-Scenes Work, Real world Example

Promises in JavaScript: Def, Creation, Call, Pros and Cons, Behind-the-Scenes Work, Real world Example

In the dynamic realm of JavaScript, promises stand as a pivotal concept, transforming the landscape of asynchronous programming. This comprehensive guide will navigate you through the intricacies of promises, covering everything from their definition and creation to the call stack and beyond.

Understanding Promises in JavaScript

Defining Promises

At its core, a promise is an object representing the eventual completion or failure of an asynchronous operation. It is a placeholder for the result of that operation, whether it’s resolving with a value or rejecting with a reason.

Creation of Promises in Javascript

Creating promises involves the use of the Promise constructor. We’ll delve into the syntax and explore various examples to solidify your understanding of how promises are initialized.

Asynchronous Programming with Promises

Basics of Asynchronous Programming

Before diving into promises, it’s essential to understand the fundamentals of asynchronous programming. We’ll explore callback functions, the event loop, and how JavaScript manages asynchronous tasks.

Resolve and Reject Methods

The resolve and reject methods play a pivotal role in the promise lifecycle. Learn how these methods determine whether a promise should be fulfilled with a value or rejected with a reason.

Chaining Promises

Chaining promises is a powerful technique for handling multiple asynchronous tasks. We’ll walk through examples, illustrating how to create a seamless flow of operations by chaining promises effectively.

Error Handling with Promises

Error handling is a critical aspect of writing robust and reliable code. Discover how promises simplify error handling and explore best practices for gracefully managing errors in asynchronous operations.

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States of a Promise: Pending, Fulfilled, and Rejected

Pending State

A promise starts in the pending state, indicating that the asynchronous operation is ongoing. Explore scenarios where promises remain in the pending state.

Fulfilled State

When a promise successfully completes its operation, it enters the fulfilled state. Learn how to handle and retrieve values from fulfilled promises.

Rejected State

In case of an error or unsuccessful operation, a promise enters the rejected state. Understand how to gracefully handle rejections and retrieve the reason for failure.

The Call Stack and Promises

Understanding the call stack is crucial for grasping how promises function. We’ll unravel the behind-the-scenes work, exploring the event loop and how promises interact with the call stack.

Pros and Cons of Using Promises


  1. Asynchronous Simplicity: Promises streamline asynchronous code, making it more readable and maintainable.
  2. Error Handling: Promises provide a structured way to handle errors, enhancing code reliability.
  3. Chaining: Learn the art of chaining promises, allowing for elegant sequencing of asynchronous tasks.


  1. Complexity for Beginners: The concept of promises can be challenging for beginners, requiring a shift in mindset.
  2. Promise Hell: Improper chaining may lead to “promise hell,” making code hard to follow.

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Behind-the-Scenes Work: Event Loop and States

Event Loop in JavaScript

Understanding the event loop is crucial for mastering promises. We’ll dissect the event loop’s role in managing asynchronous operations and maintaining the smooth execution of your code.

Pending, Fulfilled, and Rejected States

Explore the three states a promise can exist in: pending, fulfilled, and rejected. Learn how these states impact the flow of your asynchronous operations.

Tips for Effective Promise Usage

  1. Chaining Promises Effectively: Master the art of chaining promises to create a seamless flow of asynchronous tasks.
  2. Error Handling Best Practices: Implement robust error handling strategies to fortify the reliability of your code.
  3. Utilizing Promise.All: Discover the power of Promise.all for handling multiple promises concurrently.

Real Word Example of Promise in Javascript

Code Explanation

  1. Array Initialization:
   const transactions = ["debit", "credit"];

This line initializes an array named transactions with two elements: “debit” and “credit.” In a banking context, these transactions could represent different types of financial operations.

  1. makeTransaction Function:
   function makeTransaction(transactions){
       return new Promise(function(resolve, reject){
               const errorMessage = "Error: No transactions found";
               const transactionId = 456;
  • makeTransaction is a function that takes an array of transactions as a parameter.
  • It returns a Promise that either resolves with a transaction ID (if transactions are present) or rejects with an error message.
  1. Promise Chaining (.then and .catch):
           return transactionId;
  • The makeTransaction function is invoked with the transactions array.
  • The first .then block logs the transaction ID if the promise is resolved.
  • The .catch block logs an error message if the promise is rejected.
  1. initiatePayment Function:
   function initiatePayment(transactionId){
       const transactionIdentifier = transactionId;
       return new Promise(function(resolve, reject){
               const paymentMessage = "Payment successful";
           const errorMessage = "Error: Payment failed";
  • initiatePayment takes a transactionId as a parameter and returns a Promise.
  • It simulates a payment process, resolving with a success message if the transaction ID is present and rejecting with an error message otherwise.

JavaScript : Execution Context ,Call Stack ,JavaScript Execution ?(Important to Understand)

  1. Promise Chaining (.then and .catch):
       initiatePayment(transactionId).then(result => console.log(result));
       return initiatePayment(transactionId);
  • The initiatePayment function is chained after the first makeTransaction promise.
  • It logs the payment result if the promise is resolved and logs an error message if the promise is rejected.
  1. updateBalance Function:
   function updateBalance(paymentStatus){
       const paymentResult = paymentStatus;
       return new Promise(function(resolve, reject){
               const balanceUpdateMessage = "Balance updated successfully";
               const errorMessage = "Error: Balance not updated";
  • updateBalance takes a paymentStatus as a parameter and returns a Promise.
  • It simulates updating the balance, resolving with a success message if the payment status is present and rejecting with an error message otherwise.
  1. Promise Chaining (.then and .catch):
       return updateBalance(paymentResult)
  • The updateBalance function is chained after the second initiatePayment promise.
  • It logs an error message if the promise is rejected.
  1. Final .then Block:
   .then(result => console.log(result))
  • This block logs the final result (either success message or error message) of the last promise in the chain.

In summary, the code simulates a series of asynchronous operations in a banking system, where a transaction is made, a payment is initiated, and the balance is updated. Each step is represented by a Promise, and the code uses .then and .catch to handle the results of these asynchronous operations.

Full Code of Banking Transaction system using Promise in Javascrip

OutPut of Above Code :

Output of banking system using promises in javascript

Trusted Reference Sources

  1. MDN Web Docs – Promises: Link

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As we conclude our journey through the promises landscape, it’s evident that mastering this asynchronous powerhouse is essential for any JavaScript developer. Embrace the pros, navigate the cons, and harness the full potential of promises to elevate your code.

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