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How Promises Work in ES6?

In this blog, I will explain How Promises Work in ES6.

In ECMAScript 6 (ES6) and later versions, promises are a mechanism for handling asynchronous operations. They provide a cleaner and more organized way to work with asynchronous code compared to traditional callback functions. Here’s an overview of how promises work in ES6.

1. Creating a Promise

You can create a promise using the Promise constructor, which takes a function as an argument. This function, commonly referred to as the “executor function,” has two parameters: resolve and reject. The resolve function is called when the asynchronous operation is successful, and the reject function is called when it encounters an error.

const myPromise = new Promise((resolve, reject) => {
  // Asynchronous operation
  // If successful, call resolve(value)
  // If there's an error, call reject(error)

2. States of a Promise

A promise can be in one of three states:

  • Pending: The initial state; the promise is neither fulfilled nor rejected.
  • Fulfilled: The asynchronous operation completed successfully, and the promise has a resulting value.
  • Rejected: The asynchronous operation encountered an error, and the promise has a reason for the failure.

3. Handling Promises

You can handle promises using the .then() and .catch() methods. The .then() method is used to handle the fulfillment of a promise, and the .catch() method is used to handle any errors that may occur.

  .then((result) => {
    // Handle successful operation
  .catch((error) => {
    // Handle error

4. Chaining Promises

Promises can be chained using the .then() method, allowing you to sequence asynchronous operations.

  .then((result1) => asyncOperation2(result1))
  .then((result2) => asyncOperation3(result2))
  .then((result3) => {
    // Final result
  .catch((error) => {
    // Handle any errors in the chain

5. Promise.all() and Promise.race()

  • Promise.all(iterable): Takes an iterable of promises and returns a new promise that is fulfilled with an array of all fulfilled values when all promises in the iterable are fulfilled.
  • Promise.race(iterable): Takes an iterable of promises and returns a new promise that is fulfilled or rejected as soon as one of the promises in the iterable is fulfilled or rejected.

These features make it easier to work with multiple asynchronous operations concurrently.

const promisesArray = [promise1, promise2, promise3];

  .then((resultsArray) => {
    // Handle the array of results
  .catch((error) => {
    // Handle any errors in any of the promises

These are the basics of working with promises in ES6. They provide a more structured way to handle asynchronous code and improve readability and maintainability in your JavaScript applications.

Further Reading

JUnit Tutorial

Examples of Using Promises in ES6

Spring Framework Practice Problems and Their Solutions

30 MCQs on JUnit

From Google to the World: The Story of Go Programming Language

Why Go? Understanding the Advantages of this Emerging Language

Creating and Executing Simple Programs in Go

20+ Interview Questions on Go Programming Language

100+ MCQs On Java Architecture

Java Practice Exercise



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