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Table of contents
1.
Introduction
2.
Understanding JavaScript Promises
3.
Introducing Promise.all()
3.1.
Behavior of Promise.all()
3.2.
Use Cases for Promise.all()
4.
Frequently Asked Questions
4.1.
What does Promise.all() do in JavaScript?
4.2.
Does Promise.all() maintain the order of promises?
4.3.
What happens if a promise is rejected in Promise.all()?
5.
Conclusion
Last Updated: Mar 27, 2024

JavaScript Promise.all() Method

Author Gunjan Batra
0 upvote
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Speaker
Anubhav Sinha
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25 Jun, 2024 @ 01:30 PM

Introduction

JavaScript's asynchronous nature allows it to execute multiple tasks without waiting for one to complete before starting another. This is handy but can pose challenges, such as synchronizing multiple asynchronous operations. 

This is where Promise.all() comes into play. 

JavaScript Promise.all() Method

This article will delve deep into Promise.all(), its use cases, behavior, and practical examples.

Understanding JavaScript Promises

To grasp Promise.all(), let's first understand JavaScript Promises. A Promise in JavaScript represents a value that may not be available yet but will be available in the future or never. Promises have three states:

  • Pending: The promise’s outcome hasn’t yet been determined.
     
  • Fulfilled: The promise has successfully resolved.
     
  • Rejected: The promise has failed.

You can create a Promise using the Promise constructor as follows:

let p = new Promise((resolve, reject) => {
  let a = 1 + 1;
  if (a == 2) {
    resolve("Success");
  } else {
    reject("Failed");
  }
});
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Introducing Promise.all()

Promise.all() is a method that takes an iterable of promises and returns a single Promise that resolves when all of the promises in the iterable have been resolved, or rejects with the reason of the first passed promise that rejects.

Here's a simple usage of Promise.all():

let promise1 = Promise.resolve(3);
let promise2 = 42;
let promise3 = new Promise((resolve, reject) => {
  setTimeout(resolve, 100, "foo");
});


Promise.all([promise1, promise2, promise3]).then((values) => {
  console.log(values);
});

This code will output [3, 42, "foo"] to the console.

 Output

Output

Behavior of Promise.all()

There are two important things to note about Promise.all():

Order of resolution: The returned promise from Promise.all() will resolve with an array of values from the input promises in the same order as the input promises. This means Promise.all() maintains the order of the promises.

Rejection behavior: If any of the input promises is rejected, Promise.all() immediately rejects with the reason from the first promise that was rejected. This "fast failing" behavior can be useful if you want to fail as soon as any of the promises fails.

Use Cases for Promise.all()

Promise.all() is most useful when you have a group of independent promises that you want to run in parallel. Here are a few examples:

Fetching data from multiple URLs: If you have an array of URLs and you want to fetch data from all of them, you can create an array of fetch promises and pass them to Promise.all().

Waiting for multiple async operations: If you have multiple async operations that can run at the same time, you can use Promise.all() to start them all at once and wait for them all to finish.

Frequently Asked Questions

What does Promise.all() do in JavaScript?

Promise.all() takes an iterable of promises and returns a single Promise that resolves when all the input promises have resolved or rejects when any of the input promises reject.

Does Promise.all() maintain the order of promises?

Yes, Promise.all() maintains the order of promises. The promise returned by Promise.all() resolves with an array of values in the same order as the input promises.

What happens if a promise is rejected in Promise.all()?

If any promise is rejected in Promise.all(), it will reject with the reason of the first promise that rejected.

Conclusion

In conclusion, Promise.all() is an extremely powerful method that makes handling multiple promises in parallel simple and straightforward. It allows us to wait for all promises to resolve and fail fast if any of the promises are rejected. This makes it an invaluable tool for managing asynchronous operations in JavaScript. As with all powerful tools, it's important to understand its behavior and use cases to use it effectively and efficiently. 

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