better-promises

JavaScript's Promise extensions you may find useful during development.

Installation

# yarn
yarn add better-promises

# pnpm
pnpm i better-promises

# npm
npm i better-promises

CancelablePromise

The CancelablePromise class provides promises that can be canceled. There are several ways to create a CancelablePromise:

import { CancelablePromise } from 'better-promises';

// Using no arguments at all. But in this case, the promise will
// never be resolved. 
const promise = new CancelablePromise();

// Using the classic Promise executor with the additional
// abort signal, which will be aborted in case, the promise
// was resolved or rejected externally.
const promise2 = new CancelablePromise((res, rej, abortSignal) => {
  // ..
});

// Using only options. All options are optional.
const controller = new AbortController();
const promise3 = new CancelablePromise({
  // Abort signal to let the promise know, the execution
  // should be aborted. If the signal was aborted, the
  // promise will be rejected with the AbortError,
  abortSignal: controller.signal,
  // Execution timeout. When timeout was reached, the
  // promise will be rejected with the TimeoutError.
  timeout: 3000
});

// Using the executor and options.
const controller2 = new AbortController();
const promise4 = new CancelablePromise((res, rej, abortSignal) => {
  // ..
}, { abortSignal: controller.signal, timeout: 3000 })

In addition to standard promise methods (then, catch, and finally), CancelablePromise introduces two new methods: reject and cancel. It also provides a static method withFn.

Passing Async Executor

Unlike JavaScript's Promise executor, the executor passed to the CancelablePromise constructor is allowed to be a function that returns a Promise. If the returned promise is rejected, the CancelablePromise will also be rejected.

The following code will not work as expected because the executor returns a promise that gets rejected:

const promise = new Promise(async (_, rej) => {
  throw new Error('Oops!');
})
  .catch(e => console.error('Handled:', e));
// Uncaught (in promise) Error: Oops!

However, the CancelablePromise class can handle this type of error:

const promise = new CancelablePromise(async (_, rej) => {
  throw new Error('Oops!');
})
  .catch(e => console.error('Handled:', e));
// Handled: Error('Oops!')

withFn

The withFn method executes a function in sync without callbacks. It accepts the function and optional settings passed to the CancelablePromise constructor.

const controller = new AbortController();
const promise = CancelablePromise.withFn((abortSignal) => {
  return 'Resolved!';
}, {
  abortSignal: controller.signal,
  timeout: 3000,
});

promise.then(console.log); // Output: 'Resolved!'

const promise2 = CancelablePromise.withFn(() => {
  throw new Error('Nah :(');
});
promise2.catch(console.error); // Output: Error('Nah :(')

const promise3 = CancelablePromise.withFn(async () => {
  const r = await fetch('...');
  return r.json();
});
// promise3 resolves with the fetched JSON body

reject

The reject method rejects the initially created promise with a given reason. It is important to note that reject applies to the original promise, regardless of any chained promises. So, calling this method, only the initially created promise will be rejected to follow the expected flow.

The expected flow is the flow when rejection was performed in the promise executor (the function, passed to the promise constructor), and then all chained callbacks (add via catch(fn)) called.

Here is the example:

const promise = new CancelablePromise();
const promise2 = promise.catch(e => {
  console.log('I got it!');
});

// Here calling promise.reject ()and promise2.reject()
// will have the same effect. We will see the log "I got it!"

A bit more real example:

const promise = new CancelablePromise((res, rej) => {
  return fetch('...').then(res, rej);
})
  .then(r => r.json())
  .catch(e => {
    console.error('Something went wrong', e);
  });

// Imagine, that we want to reject the promise for some reason
// and stop the execution. Calling the "reject" method we expect
// the "rej" argument in the executor to be called, and then
// call the "catch" method callback.

promise.reject(new Error('Stop it! Get some help!'));
// 'Something went wrong', Error('Stop it! Get some help!')

cancel

This method rejects the promise using CancelError.

import { CancelablePromise, CancelError } from 'better-promises';

const promise = new CancelablePromise().catch(e => {
  if (CancelError.is(e)) {
    console.error('Canceled');
  }
});
promise.cancel();

ManualPromise

The ManualPromise class extends CancelablePromise and introduces the resolve method to resolve the promise manually.

import { ManualPromise } from 'better-promises';

const promise = new ManualPromise();
promise.resolve('Done!');
promise.then(console.log); // 'Done!'

It also notifies the executor about promise resolution, allowing developers to stop execution if resolved externally.

import { ManualPromise, isResolved } from 'better-promises';

const promise = new ManualPromise(async (res, rej, signal) => {
  // Imitate something async here.
  await new Promise(res => setTimeout(res, 1000));

  if (isResolved(signal.reason)) {
    // It means that ManualPromise was resolved outside. 
    // We probably want to stop executing the function 
    // as long as the result will not affect anything.
    return;
  }

  // Otherwise keep doing what we do.
});

promise.resolve('I got the result from somewhere else');