p-cancelable

What is p-cancelable?

The p-cancelable package provides a way to create cancelable promises in JavaScript. This is particularly useful for managing asynchronous operations that may need to be canceled before they complete, such as HTTP requests, long-running computations, or any other task that might not need to continue executing if certain conditions are met.

What are p-cancelable's main functionalities?

Creating a cancelable promise

This feature allows the creation of a new cancelable promise. The promise can be canceled at any time before it resolves or rejects, which triggers the onCancel function where cleanup operations like clearing timeouts or aborting requests can be performed.

const PCancelable = require('p-cancelable');

const myCancelablePromise = new PCancelable((resolve, reject, onCancel) => {
  const timer = setTimeout(() => {
    resolve('Done!');
  }, 2000);

  onCancel(() => {
    clearTimeout(timer);
    reject(new Error('Operation canceled.'));
  });
});

myCancelablePromise.cancel();

Other packages similar to p-cancelable

bluebird

Bluebird is a comprehensive promise library that includes support for cancelable promises. Compared to p-cancelable, Bluebird offers a broader set of promise-related utilities, such as map, reduce, and filter for promises, making it suitable for more complex scenarios.

abort-controller

The abort-controller package provides a way to abort one or more Web API tasks, such as Fetch API requests, by using an AbortSignal. While it serves a similar purpose for canceling tasks, it is specifically designed to work with the Fetch API and other APIs that support aborting, unlike p-cancelable which is more generic.

README

p-cancelable

Create a promise that can be canceled

Useful for animation, loading resources, long-running async computations, async iteration, etc.

If you target Node.js 15 or later, this package is less useful and you should probably use AbortController instead.

Install

$ npm install p-cancelable

Usage

const PCancelable = require('p-cancelable');

const cancelablePromise = new PCancelable((resolve, reject, onCancel) => {
	const worker = new SomeLongRunningOperation();

	onCancel(() => {
		worker.close();
	});

	worker.on('finish', resolve);
	worker.on('error', reject);
});

(async () => {
	try {
		console.log('Operation finished successfully:', await cancelablePromise);
	} catch (error) {
		if (cancelablePromise.isCanceled) {
			// Handle the cancelation here
			console.log('Operation was canceled');
			return;
		}

		throw error;
	}
})();

// Cancel the operation after 10 seconds
setTimeout(() => {
	cancelablePromise.cancel('Unicorn has changed its color');
}, 10000);

API

new PCancelable(executor)

Same as the Promise constructor, but with an appended onCancel parameter in executor.
Cancelling will reject the promise with PCancelable.CancelError. To avoid that, set onCancel.shouldReject to false.

const PCancelable = require('p-cancelable');

const cancelablePromise = new PCancelable((resolve, reject, onCancel) => {
	const job = new Job();

	onCancel.shouldReject = false;
	onCancel(() => {
		job.stop();
	});

	job.on('finish', resolve);
});

cancelablePromise.cancel(); // Doesn't throw an error

PCancelable is a subclass of Promise.

onCanceled(fn)

Type: Function

Accepts a function that is called when the promise is canceled.

You're not required to call this function. You can call this function multiple times to add multiple cancel handlers.

PCancelable#cancel(reason?)

Type: Function

Cancel the promise and optionally provide a reason.

The cancellation is synchronous. Calling it after the promise has settled or multiple times does nothing.

PCancelable#isCanceled

Type: boolean

Whether the promise is canceled.

PCancelable.CancelError

Type: Error

Rejection reason when .cancel() is called.

It includes a .isCanceled property for convenience.

PCancelable.fn(fn)

Convenience method to make your promise-returning or async function cancelable.

The function you specify will have onCancel appended to its parameters.

const PCancelable = require('p-cancelable');

const fn = PCancelable.fn((input, onCancel) => {
	const job = new Job();

	onCancel(() => {
		job.cleanup();
	});

	return job.start(); //=> Promise
});

const cancelablePromise = fn('input'); //=> PCancelable

// …

cancelablePromise.cancel();

FAQ

Cancelable vs. Cancellable

In American English, the verb cancel is usually inflected canceled and canceling—with one l.
Both a browser API and the Cancelable Promises proposal use this spelling.

What about the official Cancelable Promises proposal?

It's still an early draft and I don't really like its current direction. It complicates everything and will require deep changes in the ecosystem to adapt to it. And the way you have to use cancel tokens is verbose and convoluted. I much prefer the more pragmatic and less invasive approach in this module. The proposal was withdrawn.

p-cancelable for enterprise

Available as part of the Tidelift Subscription.

The maintainers of p-cancelable and thousands of other packages are working with Tidelift to deliver commercial support and maintenance for the open source dependencies you use to build your applications. Save time, reduce risk, and improve code health, while paying the maintainers of the exact dependencies you use. Learn more.

Related

• p-progress - Create a promise that reports progress
• p-lazy - Create a lazy promise that defers execution until .then() or .catch() is called
• More…