New Async API Promise.withResolvers Simplifies Your Code

Learn the simplicity Promise.withResolvers brings to JavaScript coding.

Photo by Mitchell Kmetz on Unsplash

JS handles asynchrony so conveniently, with early callbacks and now Promise. By this year 2024, it has evolved a new syntax for Promise — Promise.withResolvers(). It is currently in the fourth stage and is supported by all major browsers. Nodejs is not supported yet. This article will show you what benefits it brings and how it can simplify setting up your code.

Understanding the Basics of Promises

Before we explore Promise.withResolvers(), let’s recap the basics of promises. A promise in JavaScript represents the eventual completion (or failure) of an asynchronous operation and its resulting value. Traditionally, to create a promise, you pass an executor function that includes resolve and reject methods, which control the outcome of the promise.

const promise = new Promise((resolve, reject) => {
  if (Math.random() > 0.5) {
    resolve("Success");
  } else {
    reject("Failure");
  }
});

This pattern, while effective, can sometimes lead to complexity when the resolution or rejection of the promise needs to occur outside the executor’s immediate scope, often leading to nested or convoluted code structures.

Introducing Promise.withResolvers

Promise.withResolvers() simplifies the creation and management of deferred promises. This method returns an object that contains a new promise and functions to resolve or reject it. These functions can be accessed directly from the returned object, providing a much cleaner and more modular approach to managing asynchronous logic.

Basic Usage

Here’s how you can use Promise.withResolvers() in a basic scenario:

const { promise, resolve, reject } = Promise.withResolvers();
​
setTimeout(() => {
  if (Math.random() > 0.5) {
    resolve("Success");
  } else {
    reject("Error");
  }
}, 1000);
​
promise.then(result => console.log(result)).catch(error => console.error(error));

In this example, resolve and reject are used in a setTimeout, demonstrating how they can be called independently of the promise constructor, offering flexibility in various programming contexts.

Advanced Use Cases

The true power of Promise.withResolvers() shines in more complex scenarios such as event handling, stream processing, or any situation where the promise’s outcome might be determined by multiple sources or at different times.

Handling Multiple Asynchronous Events

Consider an application that listens to a series of data points from a network source and needs to perform an action once enough data has been received or if an error occurs in the stream. Here’s how you can manage this with Promise.withResolvers():

function handleDataStream(eventStream, requiredEvents) {
  const { promise, resolve, reject } = Promise.withResolvers();
  let events = [];
​
  eventStream.on('data', (data) => {
    events.push(data);
    if (events.length >= requiredEvents) {
      resolve(events);
    }
  });
​
  eventStream.on('error', (error) => {
    reject(error);
  });
​
  return promise;
}
​
const dataStreamPromise = handleDataStream(someEventStream, 5);
​
dataStreamPromise.then(data => console.log('Data Received:', data))
                 .catch(error => console.error('Stream Error:', error));

This approach allows for a clean separation of concerns, where the promise management is decoupled from the event handling logic, making the code easier to read and maintain.

Calling Promise.withResolvers() on a Non-Promise Constructor

Another interesting and advanced feature is its use with non-standard Promise constructors. This method is general enough to apply to any constructor that follows the standard Promise constructor signature.

class NotPromise {
  constructor(executor) {
    // The "resolve" and "reject" functions behave nothing like the native
    // promise's, but Promise.withResolvers() just returns them, as is.
    executor(
      (value) => console.log("Resolved", value),
      (reason) => console.log("Rejected", reason),
    );
  }
}

const { promise, resolve, reject } = Promise.withResolvers.call(NotPromise);
resolve("hello");
// Logs: Resolved hello

Conclusion

The Promise.withResolvers() method offers a more elegant and practical way to handle deferred promises in JavaScript. By abstracting the creation and control of promises away from the executor function, it not only reduces boilerplate code but also enhances code clarity and maintainability.

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