Yet another cancellation promise module

createCancellable.js

// @flow

type CancellablePromise<Result> = {
  promise: Promise<Result>,
  cancel: (reason?: string) => void
};

// $FlowFixMe: can not add custom properties to Error
const setCancelled = (error, cancelled) => Object.assign(error, {cancelled});

const createError = reason => setCancelled(new Error(reason), true);

const createControllerCancel = reject => reason => reject(createError(reason));

const createController = (): CancellablePromise<*> => {
  let cancel;

  const promise = new Promise((resolve, reject) => {
    cancel = createControllerCancel(reject);
  });

  return {
    // $FlowFixMe: flow is not detecting the assignment inside the promise
    cancel,
    promise
  };
};

const createCancellable = <Result>(
  promise: Promise<Result>
): CancellablePromise<Result> => {
  const controller = createController();

  return {
    cancel: controller.cancel,
    promise: Promise.race([
      controller.promise,
      promise.catch((error: Error) => {
        throw setCancelled(error, false);
      })
    ])
  };
};

export default createCancellable;

export type {CancellablePromise};