Can Promises Be Cancelled? A Deep Dive into Cancellation Mechanisms in JavaScript

Promise - is it possible to force cancel a promise

ES6 promises themselves do not support cancellation. However, there is work in progress to add cancellation to promises in the ECMAScript standard.

Promise cancellation proposal has been made in the WHATWG specifications to be included in the upcoming ES2021 version of JavaScript. This proposal introduces a new AbortController interface that can be used to cancel promises.

<br>const abortController = new AbortController();<br>const promise = fetch('https://example.com', {<br>  signal: abortController.signal<br>});</p><p>abortController.abort();<br>

In modern JavaScript, you can use the AbortController to cancel Promise based tasks, such as fetch requests or other async operations.

However, if you are unable to use AbortController, it is possible to implement your own cancellation mechanism for user initiated operations, such as in case of an async form submission or a network request.

Creating a custom cancellation mechanism in JavaScript: You can implement a simple cancellation mechanism by:

• Creating a CancellationToken that will allow you to cancel the operation.


• Passing the CancellationToken to the async task that you want to cancel.


• Cancelling the CancellationToken when the user initiates cancellation.

The CancellationToken can be implemented as a Promise with a cancel() method that rejects the Promise. Here's an example:

<br>const cancellationToken = new Promise((resolve, reject) => {<br>  return {</p><pre class="brush:php;toolbar:false">cancel: () => reject(new Error('Operation has been cancelled.'))

}
});

Then you can pass the cancellationToken to the async task you want to cancel, like this:

<br>const asyncTask = (cancellationToken) => {<br>  // Implement your async task here<br>  return fetch('https://example.com', {</p><pre class="brush:php;toolbar:false">cancel: () => reject(new Error('Operation has been cancelled.'))

});
}

And finally, you can cancel the async task by calling the cancel() method on the cancellationToken, like this:

<br>cancellationToken.cancel();<br>

Using a third-party library: If you want to use a third-party library to implement cancellation, you can use a library like bluebird or rxjs. Here's an example using bluebird:

<br>const Promise = require('bluebird');<br>const cancellationToken = new Promise.CancellationToken();</p><p>const asyncTask = () => {<br>  return fetch('https://example.com', {</p><pre class="brush:php;toolbar:false">signal: cancellationToken.signal

});
}

cancellationToken.cancel();

These are just a few ways to implement cancellation in JavaScript. The best approach will depend on your specific needs.

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