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

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. 🙂