JavaScript "CancellationToken" for cancelling Async/Promise functions

CancellationTokenSource.js

const CANCEL = Symbol();

class CancellationToken {

  constructor() {
    this.cancelled = false;
  }

  throwIfCancelled() {
    if (this.isCancelled()) {
      throw "Cancelled!";
    }
  }

  isCancelled() {
    return this.cancelled === true;
  }

  [CANCEL]() {
    this.cancelled = true;
  }
  
  // could probably do with a `register(func)` method too for cancellation callbacks

}

export default class CancellationTokenSource {

  constructor() {
    this.token = new CancellationToken();
  }
  
  cancel() {
    this.token[CANCEL]();
  }
  
}

usage.md

Concept inspired by the CancellationTokenSource and CancellationToken used in C# for cancelling async functions.

Example usage in an async function:

import CancellationTokenSource from './CancellationTokenSource'

async function doStuff(foo, cancellationToken) {
  // do stuff with foo on each significant step call:
  cancellationToken.throwIfCancelled();
  // for example, my use-case is uploading chunks of a file and cancelling the XHR
  return ':)';
}

var cst = new CancellationTokenSource;

doStuff('eee', cst.token)
  .then(result => console.log('result', result))
  .catch(err => console.error('failed', err));
  
setTimeout(() => cst.cancel(), 1000); // auto-cancel after one second

Or.. in a Promise:

import CancellationTokenSource from './CancellationTokenSource'

function doStuff(foo, cancellationToken) {
    return new Promise((resolve, reject) => {
      // do stuff with foo on each significant step call:
      if (cancellationToken.isCancelled()) return reject('Cancelled');
      // for example, my use-case is uploading chunks of a file and cancelling the XHR
      resolve(':)');
  });
}

var cst = new CancellationTokenSource;

doStuff('eee', cst.token)
  .then(result => console.log('result', result))
  .catch(err => console.error('failed', err));
  
setTimeout(() => cst.cancel(), 1000); // auto-cancel after one second

OK, how to do something like this:

Backstory: Dialog appears asking the user to tap a NFC chip, user clicks back button and the dialog goes away. When the dialog appears it will run a "Promise writeMessage().then(...)" method which should be cancelled when going back.

@danharper I think you are looking for the AbortController
https://davidwalsh.name/cancel-fetch

@ritteratzuken That didn’t exist 6 years ago ;) But yeah, now that it’s in the standard that’s the way to go.

@danharper I wrote a version that encapsulates the AbortController inside a Promise so it can be used natively with async/await. We use it internally at the company I work at to terminate async tasks all the way through our state management into the network request and it seems to working well so far :)

Let me know what you think.

https://gist.github.com/andrewcourtice/ef1b8f14935b409cfe94901558ba5594

That cancelled property of the CancellationToken class looks like it could be publicly set to me. So the symbol protection for the cancel() method seems not very effective. I think that should be a private field. Or in good old days before there were classes, a variable in the function closure.

But I'm going to use the AbortController as suggested above. Thanks anyway for providing this connection as I was looking for a CancellationToken for JavaScript. 🙂