The `Promise.all()` Function in JavaScript - onlinecode

The `Promise.all()` Function in JavaScript

The `Promise.all()` Function in JavaScript

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

The Promise.all() function converts an array of
promises into a single promise that fulfills when all the promises
in the original array fulfill.

Here’s an example of using Promise.all() to wrap an array of promises:

// 'p1' is immediately fulfilled with value '1'
const p1 = Promise.resolve(1);
// 'p2' will be fulfilled with value '2' after 100ms
const p2 = new Promise(resolve => setTimeout(() => resolve(2), 100));

const pAll = Promise.all([p1, p2]);
pAll instanceof Promise; // true

const arr = await pAll;

Array.isArray(arr); // true
arr[0]; // 1
arr[1]; // 2

Parallel Execution of Async Functions

When used with async functions, Promise.all() lets you execute code in parallel. Pass an array of async function calls to Promise.all(), and
JavaScript will execute the async functions in parallel.

Suppose you have two async functions getName() and getAge(). Here’s
how you can use Promise.all() to execute them in parallel:

async function getName() {
  await new Promise(resolve => setTimeout(resolve, 200));

  return 'Jean-Luc Picard';

async function getAge() {
  await new Promise(resolve => setTimeout(resolve, 200));

  return 59;

const start =;
const [name, age] = await Promise.all([getName(), getAge()]);
const end =;

name; // 'Jean-Luc Picard'
age; // 59

end - start; // Approximately 200

Error Case

If one of the promises rejects, the promise returned by Promise.all()
rejects immediately with the same error.

const success = new Promise(resolve => setTimeout(() => resolve('OK'), 100));
const fail = new Promise((resolve, reject) => {
  setTimeout(() => reject(new Error('Oops')), 100);

try {
  await Promise.all([success, fail]);
} catch (err) {
  err.message; // 'Oops'

Note that, since promises are not cancellable, each individual promise
continues execution, even if one of them errors out. If you pass an
array of async functions and one of the async functions throws an
error, Promise.all() will reject immediately with that error.
But the other functions will continue executing.

let finished = false;

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

  finished = true;
  return 'OK';
const fail = async function() {
  await new Promise(resolve => setTimeout(resolve, 10));
  throw new Error('Oops');
try {
  await Promise.all([success(), fail()]);
} catch (err) {
  err.message; // 'Oops'

  // 'Promise.all()' fails fast because 'fail()' threw an
  // error, but 'success()' is still running.
  finished; // false

  // If you wait, 'success()' will set 'finished'
  await new Promise(resolve => setTimeout(resolve, 100));
  finished; // true

Getting Fancy with Generators

The Promise.all() function doesn’t limit you to arrays, the first parameter
can be any JavaScript iterable. Arrays are iterables,
and so are generator functions. Which means you can pass in a generator
that yields promises, and Promise.all() will bundle all the yielded
promises into a single promise.

const generatorFn = function*() {
  for (let i = 1; i <= 5; ++i) {
    yield Promise.resolve(i);

const arr = await Promise.all(generatorFn());

arr; // [1, 2, 3, 4, 5]

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JavaScript Fundamentals forĀ  The `Promise.all()` Function in JavaScrip

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.
  • 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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