cancellable.js

/**
 * Implement cancellable function.
 * @param {Generator} generator - A generator that yields promises, and returns a value or throws an error when finished.
 * @returns {[Function, Promise]} A tuple `[cancel, promise]`. Calling `cancel()` throws "Cancelled" into the generator if it hasn't finished. `promise` resolves with the generator's return value, or rejects with whatever error propagates out of it (including an uncaught "Cancelled").
 */

function cancellable(generator) {
  let cancelRequested = false;
  let cancelDelivered = false;

  let cancel = () => {};
  const cancelPromise = new Promise((resolve, reject) => {
    cancel = () => {
      cancelRequested = true;
      reject("Cancelled");
    };
  });
  cancelPromise.catch(() => {});

  const promise = (async () => {
    let next = generator.next();
    while (!next.done) {
      try {
        const value = cancelDelivered
          ? await next.value
          : await Promise.race([next.value, cancelPromise]);
        next = generator.next(value);
      } catch (e) {
        if (!cancelDelivered && cancelRequested) {
          cancelDelivered = true;
        }
        next = generator.throw(e);
      }
    }

    return next.value;
  })();

  return [cancel, promise];
}

module.exports = { cancellable };