Understanding `new Promise` in JavaScript

Understanding `new Promise` in JavaScript

Understanding `new Promise` in JavaScript

In this post, we will give you information about Understanding `new Promise` in JavaScript. Here we will give you detail about Understanding `new Promise` in JavaScript And how to use it also give you a demo for it if it is necessary.

The Promise constructor takes a single parameter, an executor function.
JavaScript then executes your executor function with 2
arguments: resolve() and reject().

function executor(resolve, reject) {
  typeof resolve; // 'function'
  typeof reject; // 'function'

new Promise(executor);

Your executor function is responsible for calling resolve() to mark
the promise as fulfilled (successful) or rejected (failed).

const success = new Promise(function executor(resolve) {

const fail = new Promise(function executor(resolve, reject) {
  reject(new Error('Oops'));

const str = await success;
str; // 'OK'

const err = await fail.catch(err => err);
err.message; // Oops

You can register an event listener for when a promise is fulfilled
or rejected using the then() function.

const success = new Promise(function executor(resolve) {
  setTimeout(() => resolve('OK'), 100);

const start = Date.now();

return success.then(function onFulfilled(str) {
  str; // 'OK'

  const elapsed = Date.now() - start;
  elapsed; // Approximately 100

Using Promises for Timeouts

You don’t need to create new promises very often. Usually, libraries like
Axios or Mongoose create promises internally and return them, so you can use
then() or await.

However, not all APIs support promises. For example, the setTimeout() function only accepts callbacks.
In order to create a promise that waits for 100ms before resolving, you
should wrap a setTimeout() call in a new Promise:

async function test() {
  // Pause the async function for 100ms
  await new Promise(resolve => setTimeout(resolve, 100));

  return 'OK';

const start = Date.now();
await test();
const elapsed = Date.now() - start;
elapsed; // Approximately 100

Wrapping Node-Style Callbacks

Some async Node.js APIs, like fs.readFile(), don’t return promises.
You also need to wrap fs.readFile() in a new Promise in order
to use it with async/await.

Make sure you handle errors! Node-style callbacks take 2 parameters: an error and a result.
If error is not nullish, you should reject the promise.

const fs = require('fs');

const p = new Promise(function executor(resolve, reject) {
  fs.readFile('./package.json', (error, result) => {
    if (error != null) {
      // Note the early return!
      return reject(error);


const pkg = JSON.parse(await p);
pkg.name; // 'onlinecode.org'

Async Executor Functions for Understanding `new Promise` in JavaScript

A common mistake is making the executor an async function.

const p = new Promise(async function executor(resolve, reject) {
  await new Promise(resolve => setTimeout(resolve, 100));

const str = await p;

The above code works fine, but it creates an unnecessary promise
(remember that async functions always return a promise!) and
looks clumsy. Since async functions always return promises,
you can always replace an async executor function with a vanilla
async function call:

async function test() {
  await new Promise(resolve => setTimeout(resolve, 100));
  return 'OK';

const p = test();

The key takeaway is that you should never make an executor function
async. There’s no need.

Async/await is the future of concurrency in JavaScript. “Mastering Async/Await”
teaches you how to build frontend and backend apps using
async/await in just a few hours.
Get your copy!

JavaScript Fundamentals for Understanding `new Promise` in JavaScript

JavaScript is a programming language that is used to create interactive web pages. It is a client-side scripting language, which means that it runs on the user’s browser. JavaScript can be used to add animation, interactivity, and functionality to web pages.

Here are some of the fundamentals of JavaScript:

  • Variables: Variables are used to store data. They are declared using the var keyword.
  • Data types: JavaScript has a variety of data types, including strings, numbers, booleans, objects, and arrays.
  • Operators: Operators are used to perform operations on data.
  • Control flow statements: Control flow statements allow you to control the order in which your code is executed.
  • Functions: Functions are blocks of code that can be reused.
  • Objects: Objects are used to store data in key-value pairs.
  • Arrays: Arrays are used to store data in a sequential order.
  • Events and event handlers: Events are notifications that are sent by the browser when something happens, such as when the user clicks on an element or moves the mouse over an element. Event handlers are functions that are called in response to an event.
  • DOM manipulation: The Document Object Model (DOM) is a tree-like structure that represents the elements of a web page. JavaScript can be used to manipulate the DOM to change the appearance or behavior of a web page.

These are just some of the fundamentals of JavaScript. There are many other concepts that you can learn as you continue to develop your skills.

Here are some resources that you can use to learn more about JavaScript:

  • Mozilla Developer Network (MDN): The MDN is a great resource for learning about JavaScript. It has a comprehensive reference guide, tutorials, and articles on a wide range of topics.
  • W3Schools: W3Schools is another great resource for learning about JavaScript. It has interactive tutorials and quizzes that can help you learn the basics of the language.
  • JavaScript.info: JavaScript.info is a website that provides in-depth tutorials on JavaScript. It also has a forum where you can ask questions and get help from other JavaScript developers.

I hope this helps!

Here are some additional tips for learning JavaScript:

  • Start with the basics: Before you start trying to build complex applications, it’s important to learn the basics of JavaScript. This includes things like variables, data types, operators, and control flow statements.
  • Practice regularly: The best way to learn JavaScript is to practice regularly. Try to find some time each day to work on JavaScript projects.
  • Get help from others: If you get stuck, don’t be afraid to ask for help from others. There are many online forums and communities where you can get help from other JavaScript developers.

With a little practice, you’ll be able to learn JavaScript and start building amazing web applications.

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