JavaScript Promise.allSettled Method

In today’s world, handling asynchronous operations efficiently is crucial for web development. JavaScript offers a powerful toolset for managing asynchronous processes through the use of Promises. Among these tools, the Promise.allSettled method stands out for its ability to handle multiple promises and provide definitive feedback on their completion status. This article will explore the Promise.allSettled method in detail, making it digestible for anyone, even if you’re starting from scratch.

JavaScript Promise.allSettled Method

I. Introduction

A. Overview of Promises in JavaScript

In JavaScript, a Promise is an object that represents the eventual completion or failure of an asynchronous operation. It allows developers to perform actions after an operation is complete, without blocking the execution of the program.

B. Importance of Handling Multiple Promises

In many scenarios, you may be dealing with multiple asynchronous operations running simultaneously, like fetching data from different APIs. Effectively managing these parallel operations is vital to ensure smooth user experience and application performance.

II. Syntax

A. Promise.allSettled() Method

The Promise.allSettled() method is used to wait for all promises to settle (either fulfilled or rejected), and returns an array of objects describing the outcome of each promise.

B. Parameters

Parameter	Description
iterable	An iterable of Promises. It can be an array of Promise objects or any other iterable that contains Promises.

C. Return Value

The method returns a Promise that resolves after all of the given promises have either resolved or rejected, with an array of objects that each describe the outcome of each promise.

III. Description

A. Explanation of How Promise.allSettled Works

The Promise.allSettled() method accepts an iterable of promises and returns a new Promise. This promise resolves once all of the input promises have settled, returning an array that contains the results of each promise:

    Promise.allSettled([promise1, promise2])
      .then((results) => {
          console.log(results);
      });
    

B. Comparison with Promise.all

Promise.all waits for all promises to be fulfilled. If one promise is rejected, it short-circuits and returns the error. In contrast, Promise.allSettled waits for all the promises to settle, whether they are fulfilled or rejected. This means you will always get an array of results with their statuses:

    Promise.all([promise1, promise2])
      .then((results) => {
          console.log(results);
      })
      .catch((error) => {
          console.error(error);
      });
    

IV. Example

A. Simple Example of Using Promise.allSettled

Let’s create a simple example that uses Promise.allSettled with two promises, where one will resolve and the other will reject:

    const promise1 = new Promise((resolve) => {
        setTimeout(() => resolve('First Promise Resolved'), 1000);
    });

    const promise2 = new Promise((_, reject) => {
        setTimeout(() => reject('Second Promise Rejected'), 500);
    });

    Promise.allSettled([promise1, promise2])
      .then((results) => {
          console.log(results);
      });
    

B. Explanation of the Example Code

In this example:

• promise1 resolves after 1 second, returning a message.
• promise2 rejects after 0.5 seconds, returning an error message.
• When Promise.allSettled is invoked, it waits for both promises to complete and then logs the results:

Status	Result
fulfilled	First Promise Resolved
rejected	Second Promise Rejected

V. Browser Compatibility

A. List of Compatible Browsers

As of October 2023, Promise.allSettled is widely supported in modern browsers:

• Chrome (≥ 76)
• Firefox (≥ 63)
• Safari (≥ 12.1)
• Edge (≥ 76)
• Node.js (≥ 11)

B. Importance of Checking Compatibility

If your application needs to support older browsers, checking compatibility is crucial. You can use polyfills or alternative methods in these cases.

VI. Conclusion

A. Summary of Key Points

In this article, you learned about the Promise.allSettled method, its syntax, and its application in handling multiple asynchronous operations. You also saw how it differs from Promise.all and explored an example that illustrates its function.

B. Final Thoughts on Using Promise.allSettled in Applications

Utilizing Promise.allSettled can significantly enhance your ability to manage asynchronous tasks in your applications. It provides a comprehensive view of all tasks, ensuring that you can handle errors gracefully while still receiving results from successfully fulfilled promises.

FAQ

Q1: What happens if I pass non-promise values to Promise.allSettled?

A1: Non-promise values will be treated as fulfilled promises. For example, if you pass a number, it will be returned as fulfilled with that number as the result.

Q2: Can I use Promise.allSettled with an empty array?

A2: Yes, if you invoke Promise.allSettled with an empty array, it will resolve immediately with an empty array as its result.

Q3: Is Promise.allSettled asynchronous?

A3: Yes, calling Promise.allSettled is an asynchronous operation. It returns a promise that will be resolved when all the promises in the iterable have settled.

Q4: When should I use Promise.allSettled instead of Promise.all?

A4: Use Promise.allSettled when you want to handle multiple promises and need to know the results for all of them, regardless of whether any were rejected. Use Promise.all when you require all promises to be fulfilled for further actions.