JavaScript Promise.withResolvers() Function (original) (raw)

Summary: in this tutorial, you will learn how to use the JavaScript Promise.withResolvers() method to create a new promise with its resolve and reject functions.

Introduction to the JavaScript Promise.withResolvers method

When creating a new Promise object, you typically pass resolve and reject functions to the promise constructor like this:

const promise = new Promise((resolve, reject) =>{ // ... });Code language: JavaScript (javascript)

This allows you to call the resolve and reject functions inside the promise constructor only.

To call these functions outside of the promise constructor, you often have to write the following boilerplate code:

`let resolve, reject;

const promise = new Promise((res, rej) => { resolve = res; reject = rej; });

Math.random() > 0.5 ? resolve("Success") : reject("Error");`Code language: JavaScript (javascript)

In this code:

• First, declare variables that hold the resolve and reject functions of the promise.
• Second, create a new Promise object using the promise constructor and assign the resolve and reject functions to these variables. This makes the resolve and reject functions available outside the promise constructor.
• Third, call the resolve and reject functions outside the promise constructor.

With the Promise.withResolvers() function, you can simplify the code like this:

`const { promise, resolve, reject} = Promise.withResolvers();

Math.random() > 0.5 ? resolve("Success") : reject("Error");`Code language: JavaScript (javascript)

In this code, the Promise.withResolvers() method returns an object that contains the following properties

• promise: a new Promise object
• resolve: a function that resolves the promise.
• reject: a function that rejects the promise.

Note that the Promise.withResolvers() has been available since ECMAScript 2024.

JavaScript Promise.withResolvers method example

The following example shows how to use the Promise.withResolvers() method to handle user input.

Suppose you have a dialog prompting a user to approve or reject a request. When the user opens the dialog, the approve and reject buttons appear.

If you don’t use a promise, you can handle the approve/reject button click events like this:

`const btnReview = document.querySelector('#btnReview');

btnReject.addEventListener('click', () => { // handle rejection dialog.close(); });

btnApprove.addEventListener('click', () => { // handle approval dialog.close(); });`Code language: JavaScript (javascript)

This code should work fine but has some downsides:

• The code to handle user interaction is spread across event handlers.
• Duplicate code for closing the dialog.

To avoid these issues, you can use a Promise.withResolvers() method.

index.html

`

Please approve/reject this request?

        <button id="btnApprove">Approve</button>
        <button id="btnReject">Reject</button>
    </dialog>
    <p id="message" hidden></p>
</body>

app.js

`const btnReview = document.querySelector('#btnReview'); const btnApprove = document.querySelector('#btnApprove'); const btnReject = document.querySelector('#btnReject'); const dialog = document.querySelector('dialog');

const { promise, resolve, reject } = Promise.withResolvers();

btnReview.addEventListener('click', () => dialog.show()); btnApprove.addEventListener('click', resolve); btnReject.addEventListener('click', reject);

promise .then(() => (message.innerHTML = 'You approved it.')) .catch(() => (message.innerHTML = 'You rejected it.')) .finally(() => { message.hidden = false; dialog.close(); btnReview.remove(); });`Code language: JavaScript (javascript)

How it works.

First, select elements of the pages including btnReview, btnApprove, btnReject, and dialog:

const btnReview = document.querySelector('#btnReview'); const btnApprove = document.querySelector('#btnApprove'); const btnReject = document.querySelector('#btnReject'); const dialog = document.querySelector('dialog');Code language: JavaScript (javascript)

Second, create a new promise with the resolve and reject functions:

const { promise, resolve, reject } = Promise.withResolvers();Code language: JavaScript (javascript)

Third, wire up the event handlers with the click events of the btnReview, btnApprove, and btnReject buttons:

btnReview.addEventListener('click', () => dialog.show()); btnApprove.addEventListener('click', resolve); btnReject.addEventListener('click', reject);Code language: JavaScript (javascript)

Finally, call the promise object with the then(), catch(), and finally() methods:

promise .then(() => (message.innerHTML = 'You approved it.')) .catch(() => (message.innerHTML = 'You rejected it.')) .finally(() => { message.hidden = false; dialog.close(); btnReview.remove(); });Code language: JavaScript (javascript)

By using the Promise.withResolvers() method, we can achieve two objectives:

• Centralizing the user interaction inside a promise.
• Removing duplicate code by moving it to the finally() method of the promise.

Summary

• Use the Promise.withResolvers() method to create a new promise with its resolve and reject functions.

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